Labour would regain Thanet South where Conservative MP Laura Sandys is to stand down - with UKIP coming second, a poll suggests.

The UKIP-commissioned survey comes days after Laura Sandys announced she would not contest the Thanet South seat at the 2015 general election.

It was carried before the MP's announcement by the polling organisation Survation.

The survey illustrates the impact UKIP could have on what has generally been considered a two-way marginal between the Conservatives and Labour.

Speculation that UKIP leader Nigel Farage might stand in the constituency has intensified in the wake of Laura Sandys' decision.

The poll showed if there was an election tomorrow, Labour would be in first place with 35% (up nearly five percentage points on the general election), UKIP second on 30% (up 24 points), the Conservatives third on 28% (down 20 points) and the Liberal Democrats fourth on 5% (down 10 points).

Of potentially greater significance is that 78% of UKIP voters in Thanet South said they would not vote Conservative even if their preferred party did not field a candidate.

That will concern the Conservatives and appears to underline the difficulties the party would have in winning back UKIP supporters and countering the threat posed by Labour.

If UKIP did not stand in the constituency, 41% said they would not vote, 22% said they would vote Conservative, 19% Labour, 4% for other parties and 13% did not know. On those figures, Labour would still win the seat.

UKIP leader Nigel Farage said in a BBC interview: "This whole analysis that the UKIP vote purely damages the Conservatives' chances simply isn't true.

"UKIP is getting itself now into a position in some of the marginals where it is a serious contender to win seats in Parliament, and not just split them."

Labour's prospective parliamentary candidate Will Scobie acknowledged the party needed to do more to win back voters who had switched.

"We have not done enough in certain areas," he said. "But the significant issue is whether the Conservatives will go for a Eurosceptic candidate to replace Laura Sandys."

Alan Bown, the UKIP donor who commissioned the poll, said: "I was sure UKIP's increasing popularity and support meant that the picture was much more complicated.

"I believed that we were taking significant numbers of votes from Labour and the Liberal Democrats as well as the Tories."