Councillors on Shetland have backed plans to build up to four tunnels to link some of the largest and most populated islands, after years of isolation and decline.

The council voted on Tuesday to investigate financing options for the first two subsea tunnels, which would link Shetland’s Mainland with the two large northerly islands of Yell and Unst.

Islanders on Yell and Unst have campaigned for years for fixed links to replace their ageing and unreliable ferries, after witnessing families leaving, businesses closing and parents forced to live away from home for work.

The two projects, the first of their kind in Scotland, will cost about £655m to build and take at least eight years to complete.

Emma Macdonald, the leader of Shetland Islands council, said it would press the Scottish and UK governments for help with the construction costs, which were unaffordable for Shetland on its own.

“Islands with fixed links repopulate, enjoy economic growth and experience a reduction in their average age,” Macdonald said. “We have no ‘do nothing’ options here. Ferries and tunnels are both needed to unlock the potential of Shetland, and both the Scottish and UK governments have a vested interest in helping that happen.”

The archipelago’s roll-on, roll-off ferries are more than 32 years old on average. They have struggled to recruit and retain staff; 50% of their crew are aged 46 or over. They stop operating overnight, have limited capacity and are routinely unable to sail due to bad weather, while facing soaring repair and replacement costs.

For many islanders, that dependency increases the sense of insecurity and isolation, and drives depopulation. Relatively short journeys can last hours.

Shetlanders often look enviously at their near neighbours in the Danish-speaking Faroes, where its tunnels, including the world’s only undersea roundabout, offer seamless connections, and the Norwegian islands linked by tunnel to the mainland.

Councillors hope to persuade Scottish and UK ministers to provide some of the core funding, either through the Scottish National Investment Bank or national wealth fund, by arguing that the islands are an essential part of the wider UK economy.

The UK’s only space port at SaxaVord on the far northern tip of Unst is due to host its first rocket launches later this year, and Shetland produces 22% of Scotland’s farmed salmon and 88% of its farmed mussels, while its trawlers land seafood valued at £147m.

Engineering consultants calculated the road tunnel between Mainland and Yell would cost about £352m to dig, with operating costs of £90m over the next 60 years.

The tunnel linking Yell with Unst would cost about £300m, with running costs of £72m. Those costs would partly be met by tolls and could be part-financed by private investors who could then run the tunnels.

In turn, each would generate tens of millions of pounds in growth, and improve the islands’ social and economic resilience, the council said. Official data shows the population has fallen by 24% in the last 40 years.

Councillors also agreed on Tuesday that tunnels could be built later to link two smaller islands that sit east of Mainland – Bressay and Whalsay, with new ferries proposed for the outlying islands of Papa Stour and Skerries.

If Shetland’s tunnels are funded, their construction is expected to reinvigorate calls for subsea tunnels or bridges in the Western Isles, which are also heavily dependent on ferries.

Orkney Islands council argues that its geography makes tunnels and bridges less significant; some islands are already connected to mainland Orkney by fixed links known as the Churchill barriers.

Alice Mathewson, a spokesperson for Yell and Unst tunnel action groups, said the tunnels would be financially viable, and lower carbon than the ferries. “These links will not only bridge geographical divides but also enhance the prosperity and wellbeing of our island communities,” she said.

The Scottish government has been approached for a response.

A herd of cows has disrupted travel for thousands of train passengers after wandering on to Britain’s busiest intercity railway line.

Trains between London and Manchester were among those cancelled, with delays and disruptions affecting multiple train services on Tuesday after the errant cows blocked the west coast mainline in Staffordshire for more than three hours.

The cow incursion, which occurred shortly before 9am, was expected to affect services until 4pm, with delays and cancellations for Avanti West Coast and Lumo services between London Euston and Scotland, as well as London Northwestern and West Midlands trains.

Tickets are being accepted on certain alternative routes.

The incident blocked the line between London and Manchester a day after the prime minister-apparent, Andy Burnham, announced plans for No 10 operations split between the two cities – undeterred by his late arrival in Westminster last week on another delayed Avanti train, in a separate, non-cow incident.

According to the Press Association, railway staff had difficulties contacting the farmer responsible for the animals to move them off the track.

Burnham has called for the UK to be “able to take greater public control” of transport, but it is unclear if this is the kind of control he had in mind.

Revised plans for HS2 should not be put into action until the government is confident they can be delivered, according to the public spending watchdog.

The project to build the high-speed railway must be put on a stable footing to avoid a repeat of past failures, the National Audit Office (NAO) said in a report.

Last month, the transport secretary, Heidi Alexander, announced that the scheme would now cost up to £102.7bn and trains would not start running between London and Birmingham until as late as 2039 – £70bn more and 13 years later than originally promised. She said the project would not be entirely completed until as late as 2043.

After a 15-month review, Mark Wild, the chief executive of HS2 Ltd, came up with a new detailed plan for the remaining work.

The NAO said a “considered approach” was being taken to resetting HS2, but added “significant work” remained before the project was completed.

The report said the Department for Transport (DfT) and HS2 Ltd were aiming to complete the reset by spring 2027, but added: “It is crucial that they get it right this time following past failures.

“They should ensure that they do not proceed with putting the plans into action until they are confident everything is in place to deliver against them.”

The NAO said the DfT and HS2 Ltd should review in autumn how “realistic” the new timetable was and revise it if necessary. It estimated the cost of the reset process would be £153m.

Constructing HS2 from London to Birmingham, as well as the now abandoned onward legs to Leeds and Manchester, was initially estimated to cost £32.7bn at 2011 prices. The latest cost estimate is roughly double the figure estimated in 2020. Services were initially scheduled to begin this year.

The Manchester HS2 leg was cancelled in October 2023 by the then prime minister, Rishi Sunak.

The NAO said most of the cost increases were caused by “cost underestimation, inefficient delivery and scope changes”.

Gareth Davies, the head of the NAO, said: “Establishing a fully robust estimate of cost and schedule, completing commercial negotiations and getting the right capabilities in place is necessary before they [DfT and HS2 Ltd] can complete the reset.”

A DfT spokesperson said: “Following years of mismanagement, this government has taken decisive action to reset HS2 and ensure the safe delivery of the line between Birmingham and London at the lowest reasonable cost.

“The reset is driving faster, more efficient construction on the ground, with six major construction milestones reached ahead of schedule last year.”

A spokesperson for HS2 Ltd said: “Fundamentally resetting HS2 was the only way to regain control of the project and break the cycle of poor delivery, delays and cost increases.

“This is a hugely complex task, requiring a vast amount of external industry expertise, and has been carried out in parallel with an increase in productivity across HS2’s vast 140-mile construction programme.

“Any costs associated with the reset will ultimately pay for themselves through improved management and efficiencies.”