By Jessica Elgot, Deputy political editor

If the exit poll is correct, Keir Starmer will enter No 10 on Friday with a commanding majority. Rayner, who would become deputy prime minister, said the party was taking nothing for granted.

“Keir has done a tremendous job in transforming the Labour party and putting forward a program for government that the country can get behind after 14 years of the chaos and the scandals and the decline we have seen under the Tories,” she told Sky News. “I think they are getting punished for that – that’s pretty clear in the polls as well.”

The Labour peer and former cabinet minister Peter Mandelson told the BBC he was “gobsmacked” by the exit poll and called the projected landslide an “extraordinary achievement for Keir Starmer and his team”. “An electoral meteor has now struck planet Earth,” he said, adding: “In a sense, it’s not surprising given everything the country has gone through in the last 10 years.”

Labour is on course to double its number of seats in parliament without a significant increase in the number of votes cast for the party, because of the distribution across the country. In seats the Tories won in 2019, Labour is up five points, and the Tories down 28.

The shadow health secretary, Wes Streeting, told the BBC he could “never have imagined that Labour could do this in one term, I thought it was impossible, at best this was a two-term project”. He dismissed suggestions that Labour had won only on the back of a Conservative and SNP collapse. “Yes, the implosion has put wind in Labour’s sails, but the only reason we have sails on the ship, and the ship is shipshape, is because Keir Starmer took the vessel from the shipwreck in 2019.”

Starmer, in a post on X published shortly after the exit polls were announced, thanked everyone who had “put their trust in our changed Labour party”.

Labour is set to regain swathes of northern England and the Midlands, and to gain in the region of 40 seats in Scotland. It is also likely to take seats it has never previously held in rural England and the south-east.

But it is also predicted to have suffered some shock casualties, including losses to Reform in two seats in Barnsley, losing Dan Jarvis and Stephanie Peacock -even though one of the Reform candidates had already been disowned by Reform and would sit as an independent.

The shadow culture secretary, Thangam Debbonaire, is also predicted to have lost her seat to the Greens in Bristol Central.

Prof Sir John Curtice, the psephologist who led the team that produced the exit poll, told the BBC its prediction for Reform’s total number of seats was its most uncertain.

Labour’s national campaign coordinator, Pat McFadden, credited Starmer’s “transformation of the Labour party” for taking it from its worst defeat since the 1935 to its best result since Tony Blair’s 1997 landslide.

“We have campaigned as a changed Labour party, ready to change Britain,” he said.

“It is remarkable that Labour was in a competitive position in this election given what happened in 2019. Whatever has been claimed throughout the campaign, the Labour party has assumed nothing about the result and has worked tirelessly to bring our message of change to people across the country.”

Bridget Phillipson, the shadow education secretary, was the first Labour MP to be elected when the first constituency was declared, in Houghton and Sunderland South. Reform beat the Conservatives into third place, giving Phillipson a majority of 7,169.

Phillipson said in her speech that the British people “chose change” and praised Starmer’s leadership. She said: “Today our country with its proud history has decided that they believe that our best days lie ahead.”

She said Labour would be a government “determined to build a Britain where background is no barrier, no matter who your parents are, to tear down the barriers to opportunity”. Phillipson appeared to acknowledge that the party would be in government, saying: “Labour will honor the trust that you have placed in us, I will honor the trust that you have placed in me and I will work every day to deliver on the promises Labour has made.”