British Prime Minister Theresa May looks on course to increase her parliamentary majority in Thursday's election, an opinion poll showed today, shortly after another survey suggested the race with the opposition Labour Party was neck and neck.

Polling firm Opinium said the lead of Mrs May's Conservative Party over Labour had widened to seven points, up by a percentage point from its previous poll published on Saturday.

The new poll was the first by a major firm that was conducted in its entirety after a deadly attack in London by Islamist militants on 3 June. The field work ran from 4-6 June.

It put support for the Conservatives at 43%, unchanged from Saturday's poll, while Labour fell one point to 36%.

"Labour's campaign surge appears to have crested as our final poll of the campaign is the first to see the Tory (Conservative) lead expand rather than contract," said Opinium's head of political polling, Adam Drummond.

"If this is the start of a trend, and it is supported by other work Opinium has done, then it is worth bearing in mind that at the last election 15% of voters made their mind up either on polling day or the day before."

Meanwhile, a model by businessman and former Conservative Party donor Michael Ashcroft showed tonight that Mrs May is on track to win a 64-seat majority.

Mr Ashcroft, who has funded a significant amount of opinion polling in recent years, said his model now pointed to a greater majority for the Conservatives than it had on 2 June, when it indicated a majority of 60.

"As always, it is important to emphasise that the Ashcroft Model deals with probabilities not predictions, meaning the actual result may well fall either side of these estimates," Mr Ashcroft said, adding that model results were sensitive to assumptions about which groups of voters turned out.

British opinion polls have shown a wide range of estimates of support for the two main political parties, adding to scepticism among many critics who hammered the industry for failing to accurately predict the outcome of the 2015 election and last year's referendum vote to exit the European Union.

However, all the polls have shown a steady narrowing of the lead of Mrs May's Conservatives over the past three weeks following the publication of the parties' election policy pledges.

Opinium had a Conservative lead of 19 points over Labour early in the campaign.

Mrs May began to struggle after she fielded 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.

In the wake of Saturday's attack in London, Opinium said its poll showed a significant rise in national security as an issue for voters – 11% said anti-terrorism policy would influence their vote, up from 2% last week.

Respondents said Mrs May was the most trusted leader on fighting terrorism and on Brexit, while Labour leader Jeremy Corbyn was most trusted on Britain's health service.

The Opinium poll made for more comfortable reading for Mrs May than another one published overnight by Survation which showed her lead over Labour had narrowed to just one percentage point.

The Conservatives had a slim 17-seat majority before parliament was dissolved for the snap election.

The poll, for ITV television, was conducted on Friday and Saturday, before the attack in London.

It echoed another Survation poll, published for the Mail on Sunday newspaper, which also gave the Conservatives a one-point lead.

No other poll has shown a lead that tight.