The Tories have been forced to stress the party's chairman, Lord Feldman, remains neutral when donors approach him over the EU referendum. The move comes after Conservatives for Britain chairman Steve Baker claimed Feldman had admitted that he directed potential fundraisers to the Britain Stronger in Europe (BSE) campaign.

"Even more troublingly, Feldman, our chairman, has confirmed to me that he has directed donors to give money to the pro-EU BSE campaign in a personal capacity and in his spare time," the Eurosceptic MP wrote in The Daily Telegraph. "He also tells me that he will point people in the direction of the Vote Leave campaign, but they are yet to receive a referral from our party chairman."

But a Conservative spokesman told IBTimes UK that Feldman, who is under pressure over the Mark Clarke bullying scandal, directs donors to the pro-EU and Brexit campaigns. "Donors will call Feldman to ask how to support both the 'in' and 'out' campaigns," the spokesman said. "Feldman will simply direct them to the relevant people on the campaigns. It is up to the individual donors themselves if they then choose to contact or support either campaign."

The latest development comes after a row broke out over whether David Cameron should give his ministers a free vote on the EU referendum. Lord Heseltine warned the prime minister against such a move, claiming the decision would trigger a civil war at the top of the party.

Cameron continues to re-negotiate with Brussels over the UK's relationship with the EU ahead of the vote, which he has promised to hold before the end of 2017. The Tory leader, among other things, wants EU leaders to agree to his welfare reform. The proposal would block EU migrants from accessing benefits in the UK until they have spent at least four years in the country.

The plan has been met with resistance by the so-called Visegrád Group, of the Czech Republic, Hungary, Poland and Slovakia. The nations have even described Cameron's proposal as "discriminatory".

But immigration remains a hot top in the UK and a recent survey from Ipsos Mori put the issue at the top of Britons' concerns. However, the latest study of more than 500 people from the pollster between 12 and 14 December found "remain" was 26 points ahead of "leave" (58% versus 32%, respectively).