Today’s YouGov election model estimates the Tories will win 308 seats, 18 short of a 326-seat majority.
Labour are estimated to take 261 seats, up from 257 on Friday, increasing the possibility of a hung parliament, according to a projection by polling company YouGov.
Theresa May’s lead has now narrowed to four points according to the model with just four days of campaigning to go.
Friday’s model suggested Mrs May’s Tories would be 13 seats short suggesting last night's Question Time election special did the Prime Minister no favours.
Today’s YouGov election model estimates the Tories will win 308 seats, 18 short of a 326-seat majority
The election model is a blow to the Prime Minister as she comes under fire from Labour and the Liberal Democrats.
Labour leader Jeremy Corbyn has claimed the Tories’ tax plans are in “chaos” after Mrs May failed to back a senior minister who said higher earners will not face an income tax hike under a new Tory government.
Defence Secretary Sir Michael Fallon said voting Conservative in the General Election on Thursday was "the only way" people could be sure income tax would not go up.
The YouGov election model estimates Theresa May will be 18 seats short of a majority
His comments went further than the party's manifesto, which said they would keep taxes "low" but did not rule out a rise in income tax.
Campaigning in West Yorkshire, Prime Minister Theresa May declined to be drawn on his comments, saying only it was their "firm intention" to reduce taxes for ordinary families.
"Our position on tax hasn't changed. We have set it out in the manifesto," she said.
"What people will know when they go to vote on Thursday is that it is the Conservative Party that always has been, is and always will be a low-tax party.
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YouGov's June 3 election model estimates the Tories will win 308 seats, falling 18 short
"It is our firm intention to reduce taxes for ordinary working families."
Her comments were seized on by Mr Corbyn, who said the Conservative leadership was in disarray.
"I think there's complete chaos going on at the top of the Government," he told reporters during a campaign visit to Lincoln.
"One minister says they're going to give no more tax rises, indeed possibly tax reductions for the very wealthiest, then they can't answer the question about tax rises for the rest of the population, then they can't answer the questions about funding social care."
Theresa May has been out on the campaign trail
Jeremy Corbyn accused the Tories of being in "chaos" over tax
For the Liberal Democrats, former business minister Jo Swinson said: "The Conservatives are in utter mayhem over their tax policy."
The Conservative manifesto said there would be no increase in VAT but dropped David Cameron's pledge not to raise income tax or national insurance contributions after Chancellor Philip Hammond complained that it limited his room for manoeuvre.
However, in an interview with The Daily Telegraph, Sir Michael made it clear that income tax "absolutely" would not rise under a re-elected Tory government.
The YouGov election model estimates the Tories will win 308 seats
Asked if high earners could confidently vote Conservative next week, safe in the knowledge that their income tax would not go up, Sir Michael said: "Yes.
"You've seen our record. We're not in the business of punishing people for getting on, on the contrary we want people to keep more of their earnings.
"The only way they can be sure their taxes won't rise is to vote Conservative. We already know your tax will go up if you vote Labour on Thursday."
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Liberal Democrat former business secretary Sir Vince Cable said that if the Conservatives were ruling out income taxes rises, they would have to find the revenue from elsewhere to meet their spending commitment.
"Michael Fallon's comments raise the obvious question as to where the Conservatives will raise the money that their Chancellor knows will be needed if promised funding for schools, the NHS, the police and defence is to materialise.
"Since they are ruling out increases in income, corporate tax and VAT, we must assume that there will be an increase in national insurance and in various 'stealth taxes' yet to be specified. It undoubtedly raises suspicions."