Insurance guru Arron Banks, who has previously given tens of thousands of pounds to the Conservatives, is to present Nigel Farage with a cheque for £100,000.
But Mr Banks was dismissed by Commons leader William Hague as “someone we haven’t heard of” whose change of allegiance would have little impact.
The move is the latest in a series of announcements designed to disrupt the Tories’ final party conference before the general election.
Over the weekend Mark Reckless followed fellow MP Douglas Carswell in jumping ship to Ukip and triggering Commons by-elections.
Former deputy mayor of London Richard Barnes has also declared he is switching allegiance.
Mr Hague insisted the switch would do nothing to overshadow Mr Cameron’s keynote speech to the party conference.
“I’ve never heard of him so we are not going to get too upset about that,” he told BBC Radio 5 Live.
“It’s certainly not going to overshadow the Prime Minister’s speech today that someone we haven’t heard of has gone to Ukip.”
Mr Hague denied the defection showed a growing haemorrhaging of support from the party.
“I don’t think so at all. The Prime Minister will make a very strong speech today bringing together all the plans we have set out for the future,” he told BBC Breakfast.
“I think the conference has fully moved on from the defection of one MP to Ukip on Saturday. The gentleman you are talking about is not a senior figure in this party, not somebody I know personally at all.
“It’s up to the voters in the end, not people who are politically active and move about between parties - which happens in all directions. When I went to Clacton two weeks ago I was welcoming a Ukip councillor back to the Conservatives.”
Mr Banks said: “We are being dragged down to the lowest common denominator of financial growth when we should be looking to trade worldwide and should be basing our future prosperity on a long-term global outlook.
“The Conservative Party try to sell us the myth that EU reform is achievable when frankly all Cameron can offer is tinkering around the edges. He is not even able to bring himself to say that he would support UK exit if his supposed reforms do not work.
“Being a member of the EU is like having a first class ticket on the Titanic. Economically, remaining in the EU is unsustainable.”
Labour frontbencher Michael Dugher said: “David Cameron is haemorrhaging support and his authority is ebbing away. He can’t control his party, who clearly have no confidence in his leadership.
“And once again we see that Ukip are reliant on Tory money as well as Tory policy and Tory politicians, putting paid to the idea that they stand up for working people.”