Jeremy Corbyn, the leader of Britain's opposition Labour Party delivers a speech in Carlisle, England, Britain, June 4, 2017. [Photo/Agencies]

But May's campaign began to struggle after she proposed a plan to make elderly people pay for more of their social care, even though she moved quickly to say there would be a limit on the amount of costs that people would face.

It remains unclear whether Saturday's attack will have an impact on the election. The Conservatives' lead over Labour continued to fall after a suicide bombing in Manchester on May 22 but polling firms have linked the narrowing to the rival parties' policy proposals rather than the attack.

Fifty percent of respondents in Monday's poll thought May would make a better prime minister than Labour leader Jeremy Corbyn but his credibility as a potential leader of the country has risen to 36 percent from 15 percent in early May.

Several other polls are due to published before voters go to the polls on Thursday.

Reuters