The Conservatives' advantage over Labour has narrowed to just five points in the first opinion poll taken since Monday night's suicide bomb attack in Manchester.
The YouGov survey for The Times, conducted on Wednesday and Thursday, put the Conservatives down a point on the previous week on 43% and Labour up three on 38%, with Liberal Democrats up one on 10% and Ukip up one on 4%.
A TNS/Kantar poll taken before Monday night's atrocity put the Conservatives down five points on 42%, with Labour up five on 34%, Lib Dems up a point on 9% and Ukip down two on 4%.
The results reflect a significant tightening in the race for the June 8 General Election, which was called in April at a point when Theresa May's party was enjoying a seemingly unassailable lead of as much as 24 points over Jeremy Corbyn's Labour.
Labour's former deputy prime minister Lord Prescott, who has given his personal endorsement to Mr Corbyn's drive for Downing Street, welcomed the findings on Twitter: "It's happening! Game on."
The YouGov poll showed a steep slump in popularity for both Mrs May and the Conservatives over the course of the campaign so far, and a sharp improvement for both Mr Corbyn and Labour.
But it suggested that voters may be returning to Tories after the bombing, with a nine-point improvement in the Prime Minister's personal favourability rating and a six-point boost for her party compared with a poll carried out immediately before the atrocity.
When the election was called, Mrs May's favourability rating - obtained by subtracting the percentage with an unfavourable opinion of her from those with a favourable one - stood at plus 10, more than 50 points ahead of Mr Corbyn on minus 42.
With a rating of minus 2, the Conservative Party as a whole was also comfortably ahead of Labour on minus 27.
By the eve of the Manchester bomb, Mr Corbyn and Labour had overhauled their rivals on the favourability ranking, with the leader's rating three points better than Mrs May's and the party two points ahead of the Tories.
But the position has now been reversed, with the Tory party now a single point clear of Labour on minus 7 to minus 8 and the Prime Minister registering a rating of plus 1 to Mr Corbyn's minus 16.
YouGov questioned 2,052 voters on May 24 and 25. TNS/Kantar interviewed 1,200 adults between May 18 and 22.
The Conservative candidate for Braintree, James Cleverly, tweeted: "Labour has thousands of Momentum activists and millions from the unions. We never thought this election was going to be a walk in the park."
The Tory candidate for North Devon, Peter Heaton-Jones, said: "So now will people believe us? Jeremy Corbyn really could be PM in just two weeks. Only way to stop him is to vote Conservative."
The Press Association's poll of polls - a rolling seven-day average of published surveys - reflected the narrowing of the gap between the Tories and Labour.
It put Mrs May's party on 46% with Labour on 35%, the gap closing to nine points where a month ago the Tories had enjoyed a 20-point lead.