David Cameron's four visits to Newark appear to have paid off according to a poll which indicates the Conservatives are on course to hold the constituency in Thursday's by-election.

Lord Ashcroft's poll puts the Tories on 42%, 15 points ahead of second-placed Ukip on 27%, with Labour trailing in third on 20%.

The results show an upturn in support for the Conservatives compared to last week's Survation poll for the Sun which put the party on 36%, with Ukip on 28% and Labour at 27%.

Lord Ashcroft said the differences shown by the two surveys could be down to his counting of "shy voters" who refuse to say or claim not to know how they will vote in relation to how they voted in the last election, with more of these likely to be Tories.

The Tory peer also released figures showing 70% of voters planning to back Ukip's Roger Helmer in the by-election are doing so in a general protest to show they are unhappy with all the parties.

The poll also showed a majority of Ukip supporters - more than six in 10 - said they were backing Nigel Farage's party to send a message that they were unhappy with their usual party.

The results emerged as the Prime Minister visited Newark for a fourth time in the campaign, telling voters he understood "temptations" to vote for other parties.

Ukip is hoping to capitalise on its success in the recent local and European elections and overturn the Conservative majority in the seat. It became vacant when former Tory MP Patrick Mercer, who had served as an independent since May last year, resigned over a lobbying scandal in April.

He stepped down amid allegations he asked questions in Parliament in return for money and did not contest the findings of a report into his conduct which suspended him from the Commons for six months.

Mr Cameron, who was on the campaign trail alongside Foreign Secretary William Hague, admitted Mr Mercer had "let down" his constituents.

"He did let you down. What he did was completely wrong," he said. "Often in by-elections people think 'Well, you let me down, why should I?' So I wanted to come here personally.

"There's always temptations to go off and vote for another party, to send a message about this or give the Government a kick about that and that's absolutely people's right, we live in a democracy.

"You choose. You're my boss - you're his boss. If you think he'd do a good job, vote for him; if you think I do a good job, vote for me at the next election. If you want to get rid of me at the next election, vote for someone else."

Mr Cameron insisted Tory candidate Robert Jenrick was committed to the Nottinghamshire constituency's long-term future unlike the other parties' representatives, who the PM claimed view the poll as a way to make gains in the run-up to the general election.

He said: "Remember all those posters years ago - a dog is for life, not just for Christmas - it is a bit the same with your member of Parliament.

"I know Robert will be here, will be working hard because he is committed to the long-term future of people in Newark and Nottingham.

"The other parties just see this as trying to make a bit of progress for something else they want to do elsewhere in the country. That is how Labour, how Ukip, how the others are addressing this by-election."

Shadow chancellor Ed Balls and deputy Labour leader Harriet Harman were also in Newark to support their candidate Michael Payne.

Ms Harman posted a picture online of the group at a supermarket checkout "buying fish for supper".

The other eight candidates standing in Thursday's election are: David Watts for the Liberal Democrats; David Kirwan from the Green Party; two independents Paul Baggaley and Andy Hayes; Lee Woods from the Patriotic Socialist Party; David Bishop from the Bus Pass Elvis Party; Dick Rodgers from Stop Commercial Banks Owning Britain's Money and Nick The Flying Brick from The Official Monster Raving Loony Party.