The Conservatives remain on course to win a parliamentary majority at Thursday’s general election, according to a major new poll – but their predicted margin of victory has been halved in the past fortnight.

Labour and Liberal Democrat gains have eaten into the Tories’ majority, according to the poll of more than 100,000 voters that came closest to predicting the 2017 result.

The Conservatives are set to win 339 seats, with Labour on 231, the Lib Dems on 15, the SNP on 41, and the Brexit Party empty-handed, YouGov’s MRP poll for The Times forecasts.

But the level of variation in the poll suggests the Tories could secure as many as 367 seats, or as few as 311, leading YouGov’s director of political research, Anthony Wells, to conclude: “Based on the model we cannot rule out a hung parliament.”

The sophisticated polling model uses more than 100,000 voter interviews over a seven-day period, combined with analysis of key demographics, past voting behaviour and other metrics to guess the likelihood of a person voting for a particular party.

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The new verdict suggests a considerable shift in voting intention since the most recent MRP poll on 27 November, which saw the Tories barrelling towards a 68-seat majority and Labour heading for a disastrous 211 seats.

YouGov’s MRP poll was alone in correctly forecasting that Theresa May would lose her majority, calculating her party would win 310 seats. This turned out to be just seven short of the reality, with the poll correctly calling the result in 93 per cent of the country.

However, in what is arguably the least predictable election in modern history, voter intentions are changing daily, casting some doubt over the week-long snapshot.

Expectations of widespread tactical voting are a further hindrance to nationwide polls, with another YouGov survey on Tuesday suggesting one in five voters intend to do so.

“The margins are extremely tight and small swings in a small number of seats, perhaps from tactical voting and a continuation of Labour’s recent upward trend, means we can’t currently rule out a hung parliament,” said YouGov’s political research manager, Chris Curtis.

“As things currently stand, there are 85 seats with a margin of error of five per cent or less.”

If the predicted result materialises, it would hand the PM the largest Tory majority since the Eighties, topping John Major’s 21-seat margin of Commons control in 1992.

For Labour, the prediction of winning 231 seats would be its worst result for more than 30 years, according to YouGov.

But the result will add to Mr Johnson’s woes as each party prepares for a final blitz across the country in the final full day of the campaign, with Tory gains from Labour reduced from 44 to 29 since the MRP poll a fortnight ago.

Labour appear to be “patching the cracks” in its so-called “red wall” of seats across the north and the midlands, YouGov said, but the Tories are still predicted to commandeer seats such as Don Valley, Bishop Auckland, Wrexham and Great Grimsby.

The party leaders will on Wednesday issue their eve-of-poll messages to voters in a last-minute bid to shore up their support. Speaking at a rally in Middlesbrough, Jeremy Corbyn is expected to ask the electorate to “save the NHS”.

“My message to all those voters who are still undecided is that you can vote for hope in this election. It’s time for a pay rise for 12 million people, for lower fares and bills and for free childcare,” he is expected to say.

“You can vote for a Labour government that will be on your side. A Labour government that will save our NHS.”

Boris Johnson drives 'Get Brexit Done' JCB at factory in Staffordshire

The party has also released figures that it says show five more years of Tory government will push health service waiting lists up to 5.8 million by the end of 2024, up from 4.4 million in 2019. The figure was 2.5 million when the Tories took office in 2010.

Shadow health secretary Jonathan Ashworth says the stats, drawn from official data, illustrate a “Tory demolition of our NHS”.

But on Tuesday Boris Johnson hammered home his party’s own simple message, telling voters that “the stalemate in parliament has held this country back for too long”.

“Instead of concentrating on your priorities, Westminster has been mired in division and deadlock,” he said.

“Politicians have spent so much time shouting at each other and devising ways of wriggling out of Brexit that they have neglected the issues people care about,” he said during a speech in Staffordshire where he brought to reality the symbolism of his statement: “It’s time to send in the bulldozers and smash through the gridlock.”

The Lib Dems will also be on familiar political terrain for the last full day of campaigning. Jo Swinson, who will be visiting a string of Tory-held target seats in Esher and Walton, Guildford and Wimbledon, will tell voters: “My message today is clear. To stop Brexit, we must stop Boris Johnson. We have one more day left to do it.

“To all my fellow Lib Dem activists up and down the country, I say this: keep going. Keep fighting for every vote. And to everyone out there who has a growing pile of Lib Dem leaflets on their kitchen table: we can win where you live.”

She is expected to say that Mr Johnson had “showed us the kind of leader he is going to be” when he refused to acknowledge a photo of a sick child forced to sleep on the floor of an understaffed NHS hospital.

Ahead of her visit to Esher and Walton, Lib Dems released a dossier of comments from Dominic Raab to bolster their audacious bid to target the foreign secretary’s true-blue seat for what they hope will be one of the most remarkable upsets in modern electoral history.

These included Mr Raab’s description of British workers as “among the worst idlers in the world”, his claim that typical food bank users were people with “a cash flow problem”, and his characterisation of some feminists as “obnoxious bigots”.

The dossier also pointed to his admission of ignorance of the UK’s reliance on the Calais/Dover link and co-authorship of a pamphlet containing calls to break up the “monolith” of the NHS – language also used by the PM as a nascent MP.

Liberal Democrat deputy leader Ed Davey said: “Dominic Raab is a right-wing ideologue and has a long and chequered history of insulting the British public. He simply cannot be trusted to represent the people of Esher and Walton or stand up for this country on the global stage.

“This Thursday’s general election gives voters the chance to unseat this man for good. Only the Liberal Democrats can take seats off the Conservatives and beat leading Brexiteers like Raab, John Redwood and Jacob Rees-Mogg.”

The latest Ipsos MORI poll carried out from 6 to 9 December shows Ms Swinson is now more unpopular than Mr Corbyn, with a -31 per cent popularity rating. Mr Corbyn’s is now -30, up eight percentage points on the previous week, building on previous improvements during the campaign.

Both leaders, however, trail Mr Johnson, who is rated -14. But the survey was largely conducted before Monday, when the prime minister was strongly criticised for his response to the photo of a sick child sleeping on the floor of a hospital.