Labour is on course for a landslide victory at the next general election but its margins are falling across the country, according to the first megapoll under new constituency boundaries to be introduced later this year.

Sir Keir Starmer’s party could win 470 seats compared with 129 for the Conservatives, the internationalist Best for Britain campaign group said.

The analysis is based on a survey by pollster Focaldata, which asked 10,140 people in Great Britain between April 20 and May 9 which party they would vote for if a general election were held tomorrow.

The findings are likely to pile pressure on Prime Minister Rishi Sunak, although he could take some comfort from predictions that Labour’s majority could be at risk across the country.

Labour’s vote share would be at about 35%, 12 percentage points ahead of the Tories, the modelling showed.

But that is lower than last autumn when polling gave Labour a 42% share.

Best for Britain chief executive Naomi Smith told a press briefing in Westminster: “Labour’s lead does look healthy but their margins are falling everywhere.”

She said Mr Sunak appears to have won back some of the Tory voters disaffected by Liz Truss and Kwasi Kwarteng’s disastrous mini-budget last year.

Luke Tryl, UK director at the More in Common group, said: “You’re in a situation where people at the moment are going to Labour by default, not because they love the Labour Party.

“If things start to get a bit better, and people aren’t convinced by that positive offer from the Labour Party, I think things become more challenging.”

Lots of factors could come together to mean that there isn't necessarily going to be a change of government

In focus groups, people used the word “broken” to describe Britain and said they were “exhausted”, Mr Tryl said.

“The party that will win the next election is the one that can convince people, or best convince people, that it’s OK to turn on the six o’clock news and not be worried.”

Ms Smith warned that the public “can’t assume that there’s going to be this landslide”.

“What we’re seeing in our data is that it is up for grabs. Lots of factors could come together to mean that there isn’t necessarily going to be a change of government.”

But even in a worst-case scenario for Labour, it would still be the biggest party in a hung Parliament, according to the analysis.

Modelling suggests Labour could get 316 seats and the Tories 286 in a scenario where right-wing party Reform UK stands aside for the Conservatives in marginal seats – as Ukip and the Brexit Party have done previously – and undecided voters break for the Tories.