Plans to change the size of HS2 trains to maximise capacity are likely to inflate costs and mean fewer seats and slower services north of Birmingham, a senior government and rail industry figure has warned.

The £2bn order for 54 high-speed trains, to be built in Britain by a joint venture of Alstom and Hitachi, is under review as HS2 Ltd seeks to cut costs and renegotiate contracts.

The order was placed by the government in 2021, before the cancellation of the northern leg of HS2 by Rishi Sunak in 2023. The 200m-long eight-carriage units were expected to double into 16-carriage trains, the size of a Eurostar. But that will now only be possible on the new line between London and Birmingham, with existing stations such as Manchester Piccadilly unable to accommodate a 400m-long train.

Limited to eight carriages, HS2 services will reduce capacity on the conventional railway line north of Birmingham, where the Pendolino trains currently operating are faster and longer – a situation the Department for Transport (DfT) and HS2 Ltd officials are seeking to avoid, potentially by ordering longer trains.

An option under consideration at HS2 is to order about 43 longer trains, at 250m-long, which could run to most stations.

However, Chris Gibb, a non-executive director of DfT Operator (DFTO), the state-owned rail operating company, has broken ranks to warn against varying the train order, which he said could pile on extra costs and still leave HS2 unable to match west coast main line fleets.

The Hitachi-Alstom trains would not be able to tilt like the Pendolinos used by Avanti West Coast, limiting their speed on curved conventional tracks, Gibb said.

He told politicians and industry representatives at the all-party parliamentary rail group in Westminster that despite his role and being a “keen advocate of HS2” for 18 years, he was speaking out in a personal capacity “because I feel obliged to offer leadership by example and to act in the public interest”.

Gibb said there could be “no doubt that if HS2 opened by replacing 11-coach Pendolinos with eight-coach trains these would be full and leave people behind on day one”.

But to change the contract, he said, would cost time and money – and each of the potential contract variations had downsides, including wasting work already done on HS2 stations and depots.

Gibb said the government should instead retain the original train order with Alstom and Hitachi, avoiding contractual penalties and delays. But it should also plan to replace the current Pendolino fleet with longer, faster, modern versions, ready for when HS2 services start running to the north around 2040.

He said this would “give a significant increase in capacity, revenue and a reduction in journey times on all routes, with no further railway construction needed for now”.

Lord McLoughlin, who was transport secretary when HS2 was voted into law, said “it’s a tragedy where we are now”, but questioned whether Gibb’s plan would also incur significant cost. Gibb said that he would “anticipate it’s a cost saving” as the railway would require fewer Pendolinos overall.

Lord Berkeley, a long-term HS2 sceptic, backed Gibb’s idea, saying: “It could be 2040 or 2050 [when services began]. Let’s have one consistent type of rolling stock so you can keep using them in any circumstances.”

Gibb said he did not know if the order would be varied. But there is widespread speculation that the contract could be amended at the long-awaited “reset” of HS2’s schedule and budget by chief executive, Mark Wild, which is now under discussion with ministers.

A spokesperson for HS2 Ltd said: “The Hitachi-Alstom joint venture is contracted to deliver a fleet of 54 new trains for HS2. No changes have been made to the original order.”

HS2 said they were still working closely with the manufacturer and the DfT to finalise train designs ahead of production. The trains will be built in Derby and Newton Aycliffe.

The DfT was approached for comment.

Drivers who choose SUVs are compounding the pothole problem, experts have warned, as research showed hundreds of thousands of people bought bigger cars to navigate damaged roads.

Scientists said the cumulative effect of increasing numbers of heavier vehicles was a contributory factor in Britain’s potholes getting worse.

SUVs made up more than half of the 2m new cars sold in the UK last year, and a smaller but growing proportion of the 7m secondhand cars sold.

Recent polling showed almost one in eight drivers in parts of the country, including London and Yorkshire, had chosen to buy an SUV or heavier car partly due to concerns about road conditions.

According to the Opinium research, carried out for Kwikfit, 6% of drivers nationwide said they had been influenced to buy or bought an SUV primarily because of the condition of roads, a proportion that doubled among those who had suffered damage to their vehicle.

However, experts said those buyers were contributing to the problem they seek to avoid; furthermore, their vehicle and would not be immune to damage.

While the primary reason for pothole formation remained the freezing and thawing of rainwater over wet winters, and the heaviest vehicles, such as lorries, were likely to cause immediate damage, the growing weight of cars was worsening road surfaces.

Dr Ali Rahman, an assistant professor of civil engineering at the University of Leeds, said “the rising prevalence of SUVs does exacerbate pothole formation, because higher axle loads increase surface stresses, crack initiation, and road wear. They contribute a secondary but growing share of the problem, especially in cities where the road network was not designed for heavier passenger vehicles.”

Prof Anna Goodman, of the London School of Hygiene and Tropical Medicine, said: “The typical SUV exerts around five times more force on the road than the typical passenger car. The dramatic shift to SUVs in the past 20 years – from 3% of cars on the road in England to over 30% – is expected to have played some role in increasing wear and tear damage.”

Prof Christian Brand, the emeritus professor in transport at Kellogg College, Oxford, said SUVs were typically 200-300kg heavier than hatchback or sedan cars. While a single truck could do as much damage as many cars, “the rapid growth in SUVs means their cumulative impact, particularly on urban roads with lighter construction, may not be negligible and is increasingly relevant for local maintenance pressures”.

The Institution of Civil Engineers has said that heavier vehicles – also including electric cars – are a factor in pothole formation and reducing the lifespan of roads.

London is considering imposing extra charges on large SUVs, mainly due to the added safety risk they pose to other road users, as well as the space they take up. Transport for London is conducting studies on their effects on safety and congestion prior to announcing possible measures.

A spokesperson for the mayor of London, Sadiq Khan, said: “As part of their research TfL will look at the full impact of the continued growth in size and weight of these large SUVs, including any impact they have on the state and condition of London’s roads.”

According to an annual industry estimate by the Asphalt Industry Alliance, a road repair trade body, the cost of fixing all potholes and local roads across England and Wales has reached a record £18.6bn.

Steve Gooding, the director of the RAC Foundation, said: “Is it any wonder people are turning to rugged off-road oriented vehicles with the shocking state of many roads? Big wheels with all-terrain tyres are a better bet for bouncing over potholes but rarely come with the smaller, lighter cars that drivers might otherwise choose as perfectly suitable for the trips they need to make.”

Jack Cousens, the head of roads policy at the AA, said: “Sadly, all vehicles are at the mercy of potholes – regardless of what someone is driving, potholes are causing damage.”

Some swifts returning to Britain to breed will be unable to access their ancestral nesting holes after they were blocked in a £7.5m refurbishment of a Derbyshire railway viaduct, campaigners say.

Nature lovers had appealed to Network Rail to unblock three holes which were among at least nine swift nesting sites on the twin viaducts at Chapel Milton, on the edge of the Peak District.

Campaigners said Network Rail had been given extensive details of the location of the nest holes, between tiny gaps in the stonework, but three were filled with mortar in February.

“It’s utterly heartbreaking,” said Deb Pitman, a swift campaigner. “Seeing footage of what happens when swifts return to a blocked nest is deeply upsetting. They repeatedly try to access their entry point, sometimes with fatal consequences.

“It is thought established breeding pairs do not successfully relocate to a new nest site. In effect, when a nest site is lost, they stop breeding,”

Network Rail began work on the refurbishment of the 160-year-old viaducts last summer while swifts were nesting. An ecological report, commissioned by Network Rail and not required by law, found no evidence of nesting swifts.

This claim was challenged by swift watchers, who assembled 38 pieces of evidence they said showed at least nine nest sites on the viaducts. Network Rail later accepted there were swift holes and said it had worked around the nesting birds.

British Transport Police’s wildlife crime team was investigating whether nesting birds may have been disturbed, after the refurbishment work was reported to the police by campaigners last summer.

A Network Rail spokesperson said: “During essential refurbishment work on the viaduct at Chapel Milton last year, our teams identified potential swift nests. We consulted ecologists and created exclusion zones to ensure adequate protection for the nesting swifts. The essential repairs to the masonry were then completed after the bird nesting season.

“We welcome the opportunity to work with local groups to make the viaduct a more welcoming habitat for swifts, including installing additional nest boxes alongside existing nest sites.”

Campaigners say preserving the tiny swift holes would have no impact on the structural integrity of the viaduct.

Jason Adshead, from the Chapel-en-le-Frith Biodiversity Group, said: “We spent a great deal of time monitoring the work at the viaduct last year and we have evidence showing where the nest sites are. It’s information we shared with Network Rail.

“We sincerely hope they will arrange for the nest holes to be reopened before the birds return in numbers. Swifts are the sight and sound of summer and we are incredibly lucky to have them here.”

Bird lovers planned to hold a vigil beneath the viaduct on Saturday in the hope that the holes would be unblocked before the swifts returned around the end of April to breed.

Britain’s swift population has declined by 66% between 1995 and 2022, largely due to the widespread loss of nesting holes in older roofs and buildings. A national campaign seeking to ensure all new homes are built with at least one hollow “swift brick” has been successful in Scotland but was rejected by the Labour government in Westminster.

Pitman said: “Swifts have been successfully breeding since around the time of the dinosaurs and yet we can’t afford them a tiny space. I refuse to accept it.”