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Deputy Prime Minister Nick Clegg's relection hopes were dealt a fresh blow as it emerged that he was trailing Labour in his own constituency.

The latest polling data from Tory peer Lord Ashcroft show that Mr Clegg is three points behind Labour in Sheffield Hallam.

Lord Ashcroft disclosed that flawed data in polls he commissioned last year had given a false impression of voters' opinions in three key seats last November, including Mr Clegg's Yorkshire stronghold.

Ukip leader Nigel Farage's chances of snatching Thanet South from the Conservatives were underplayed, showing him behind by five points when the margin was in fact only one - by 33 per cent to 32 per cent.

And the suggestion that Ed Miliband's 26-point 2010 general election lead in Doncaster North had been squeezed to 12 points was also false as it had in fact been extended to 30 over his nearest challenger, Ukip.

Lord Ashcroft, who has become a prominent provider of polling in the run-up to the general election, said he would stop using the firm behind the numbers.

"I have not been in the habit of naming the polling companies I use, all of which are members of the British Polling Council, and I will not be naming this one," he wrote on his website.

"But I cannot allow this episode to cast doubt on the reliability of my polling more generally.

"So I must disclose that these three surveys last November are the first and only I have commissioned from a well-known but relatively new polling firm. And no, I won't be using them again."

The corrected data is:

* Sheffield Hallam: Labour 30 per cent, Lib Dem 27 per cent, Conservative 19 per cent, Ukip 13 per cent, Green 10 per cent;

* Thanet South: Conservative 33 per cent, Ukip 32 per cent, Labour 26 per cent, Lib Dem 4 per cent, Green 3 per cent;

* Doncaster North: Labour 55 per cent, Ukip 25 per cent, Conservative 13 per cent, Lib Dem 4 per cent, Green 2 per cent.