Labour is braced for record-breaking losses in Thursday’s local elections in England, which could be decisive for Keir Starmer’s future as prime minister.

In a message to voters on Thursday, Starmer said Reform’s Nigel Farage and the Greens’ Zack Polanski were “not fit to meet this moment of great global instability” and that only Labour was putting the national interest first.

“Today when you put your vote in the ballot box, you face a clear choice,” he said. “Progress and a better future for the community you call home, with a Labour council working with a Labour government. Versus the anger and division offered up by Reform or empty promises from the Greens.

“In tough times, you need politicians who will always stand up for you and your family. Time and again Nigel Farage and Zack Polanski have shown they are not fit to meet this moment of great global instability. Today I pledge firmly to you: whatever the pressure, Labour will always back you and your family and we will never waver from doing what is in Britain’s national interest.”

MPs told the Guardian that any result in which the party lost more than 1,500 council seats would be existential. But polling experts have said significantly worse results are possible – including the University of Oxford’s Stephen Fisher who has predicted the party will lose more than 75% of its seats, or about 1,900.

Labour hopes to be able to point to early holds in London, including Westminster and Wandsworth, which were traditionally Tory councils, to set the narrative that results have not been as bad as predicted.

But that message is unlikely to endure given that Reform is expected to take historically Labour councils such as Barnsley and Sunderland, while the SNP looks set for a fifth victory in the Scottish parliament elections, where Labour is also likely to lose ground, having once been on course to take power.

YouGov’s final MRP model of the 2026 Holyrood election suggests the SNP will fall just short of the 65 seats needed for a majority in the 129-member Scottish parliament, probably needing a coalition with the independence-backing Scottish Greens. Labour is tied for second place with Reform and the Greens in most polling and predicted to lose five of its MSPs.

In elections to the Welsh Senedd, Labour is on course to lose power to Plaid Cymru and record its worst ever result. Peril for Starmer could come if the Welsh first minister, Eluned Morgan, who may lose her seat, calls for him to quit on the back of the election, following Scottish Labour’s Anas Sarwar who did so in February.

MPs hoping to see a change of leadership believe that regional mayors and council leaders – among them Greater Manchester’s Andy Burnham and even the mayor of London, Sadiq Khan – may join calls for a change of prime minister. Allies of the mayors said an immediate call for resignation was unlikely.

But Starmer’s position may be safeguarded by leftwing MPs who want to see Burnham return to the Commons before a challenge. Other potential leadership contenders – Wes Streeting and Angela Rayner – are said to be unwilling to be the first to challenge Starmer.

Starmer is said to be weighing up setting out the next step in the government’s direction before the king’s speech next Wednesday. Over the coming days, MPs from across the party will hope to shape the narrative for the coming months, with some expected to call for a more radical economic vision.

On Tuesday, MPs from the centrist Labour Growth Group will launch a report, An Honest Day – A New Economic Settlement for Britain, calling for stronger government action on living standards and housing, as well as proposals on regulation, investment and state capacity.

Public research for the report will show that high numbers of those leaving Labour for the Greens and Reform favour a more radical approach to what they view as deeper structural problems than the government has identified.

“The message tomorrow will be the same one the country has been sending for years,” a source from the group said. “People keep working harder for less, watching the basics of a decent life slip out of reach and thinking ‘this system isn’t built for me’. The scale of the results will show how impatient voters are now.

“The question afterwards is whether we are finally prepared to face up to it and confront what’s gone wrong. If we aren’t, we will lose this country to the populists for a generation.”