The Conservatives are badly trailing Labour in two of their key Labour-held target seats, and are effectively neck and neck with the UK Independence party there, according to polling conducted by Survation and commissioned by a Ukip donor.

The polls show Labour with an 18-point lead over Ukip in second place in Great Grimsby and with a 20-point lead over the Conservatives in Dudley North. Great Grimsby is the Tories' 10th target seat and Dudley North their ninth.

The polling suggests that 70% of the Ukip vote is not coming from Conservative voters in the 2010 election.

In Great Grimsby the poll shows Labour on 40% (33% in the 2010 election), Conservatives 20% (31%), Liberal Democrats 13% (22%), and Ukip 22% (6%). In Dudley North there is also a Tory collapse with Labour on 45% (39% in the 2010 election), Conservatives 25% (37%), Liberal Democrats 2% (11%) and Ukip 23% (9%). These figures exclude those who say they are unlikely to vote, or those undecided.

The finding that the Conservatives are trailing in key marginals will concern Tory HQ.

Based on a uniform swing from national polling results, the Tories would expect to be on about 30% in these seats, down four points from 2010. In fact, Survation points out, the Tories are down 11 points on 23%. Meanwhile Ukip is significantly outperforming its projected figure from most national polls, up 15 points on 23%, far above the 15% projected from national polling.

Survation suggests: "This may be due to the fact that in marginal seats voters are by definition more volatile in changing their allegiance, but might also be partly due to the fact national polling from certain opinion polling companies underestimates the level of Ukip support (and over-estimates Conservative support).

The polling was commissioned by Ukip donor Alan Bown and is one of a series to look at how Ukip is faring in key marginals.

The Conservatives are likely to challenge the value of constituency polls or say many of the undecided are likely to come to the Conservatives in the next 15 months.

But Bown says his polling challenges the findings of polling by Lord Ashcroft, the former Conservative deputy chairman, suggesting Ukip is weakening Tory chances of an overall majority, and is only likely to put Ed Miliband into Number 10.

Bown said: "I believed that Ukip's popularity and recent phenomenal growth meant that in our strongest areas our support was likely to be significantly higher than Ashcroft's figure of 10-14% nationally would suggest. In addition I felt the fact that Ukip has generally in the past not been prompted for in many opinion polls may have further underestimated our support."

He said the polls he had commissioned in Great Grimsby and Dudley North show Ukip is making significant inroads in these Labour-held areas in the Midlands and North. "Whatever the Conservative Party may think about these seats being high up on their target list, their hopes of winning these seats in 2015 look like little better than a pipe-dream, based on this polling."

Survation said removing Ukip from the equation would not succeed in restoring Conservative fortunes in these areas. If Ukip ceased to exist and all Ukip defectors from the other three main parties were returned to the parties they voted for in 2010, the Conservatives would still be trailing Labour in the two seats 34% to 52%. Survation said the sample size was 1,076 total respondents (550 Great Grimsby / 526 Dudley North). The fieldwork dates were 18-22 October (Great Grimsby) and 22-24 October (Dudley North).