Ukip's vote is likely to be squeezed between now and May, pollsters believe

Ukip is also trailing in key seats in Cambridgeshire, Essex and Kent

Party is behind in Ukip stronghold of Boston and Skegness, Lincolnshire

Ukip's hopes of a significant breakthrough at the general election have been dealt a blow by polling showing it is not on course to win any of a series of key target seats.

Detailed constituency polling commissioned by Lord Ashcroft in Ukip targets found the party behind the Conservatives in all of them.

They include Boston and Skegness, where Ukip won its biggest majority in last year's council elections.

Scroll down for video

Polling by Tory peer Lord Ashcroft in four Ukip target seats found the party behind the Tories in all of them

Ukip Leader Nigel Farage receives a Whitstable Bay Beer from local landlady Aileen O'Leary during a walkabout in Sandwich, Kent

With pollsters suggesting the Eurosceptic party's vote is likely to be squeezed between now and May, the findings suggest it will struggle to win more than the two seats it already holds.

Ukip strategists have privately set a target of winning at least six constituencies in order to hail a major breakthrough.

In Boston and Skegness, identified as Ukip's best chance of gaining a seat at May's election, the party is polling 35 per cent, three points the Tories, on 38 per cent.

In North East Cambridgeshire, where the Tories have also been alarmed by Ukip's success in local elections, the party is a commanding 21 points ahead.

In South Basildon and East Thurrock, despite a 16 per cent swing from the Conservatives to Ukip, the former is six points ahead. Ukip's best hope appears to be in Castle Point, where former Conservative MP Bob Spink defected to Ukip, becoming the party's first MP. Lord Ashcroft's polling put the party just one point behind the Tories.

He has previously polled Thanet South, where Nigel Farage is bidding to become an MP, and also found Ukip one point behind.

Detailed constituency polling commissioned by Lord Ashcroft in Ukip targets found the party behind the Conservatives in all of them

David Cameron received a boost today after polls showed the Tories ahead in key marginals Boston and Skegness, North East Cambridgeshire, South Basildon and East Thurrock and Thanet South

Ukip insiders predict that Mr Farage will be ousted as party leader if he fails to become an MP but Clacton MP Douglas Carswell, who defected from the Conservatives, hangs onto his seat.

Lord Ashcroft said that over the marginal seats as a whole, 22 per cent of 2010 Conservative voters said they intended to vote for Ukip in their constituency in May.

However, the vote appears soft – with only 51 per cent of Conservative-Ukip switchers said they ruled out going back to the Tories by election day.

More than one fifth - 21 per cent - of 2010 Labour voters also said they would switch to Ukip, as did 23 per cent of 2010 Lib Dems.

Most voters in these constituencies were optimistic about the economy over the next year, both for the country as a whole and themselves and their families.

However, there were some striking contrasts: 83 per cent of Conservative voters expected Britain's economy to do well overall, compared to 48 per cent of those who said they would vote Ukip.

Two thirds of voters in these seats said they either that they were satisfied with David Cameron as Prime Minister - 35 per cent - or that they were dissatisfied but preferred him to Ed Miliband, on 31 per cent.

The proportion preferring Mr Cameron included 67 per cent of Ukip voters and nine out of ten Conservative-Ukip switchers.

When it came to the election result, 44 per cent of Ukip voters said they wanted to see the Conservatives in government either alone - 31 per cent - or in coalition - 13 per cent - while just over three in ten preferred to see Labour in office, 22 per cent with an overall majority.