UKIP’s biggest donor has warned the party could be on its deathbed, saying he is struggling to back any of the remaining leadership candidates.

Arron Banks, who has given the cash-strapped party over £1 million in the past and contributed approximately £7.5 million to the Brexit campaign, called for key figures to be removed before he gives any further money.

“It is a sad day for Ukip and perhaps the end, for this was a party meant to stand against everything that was dishonest in the political system,” he said in an email to supporters of grassroots campaign group Leave.EU.

He laid the blame for the party’s woes on a “fifth column” of “career politicians” including leadership candidate Suzanne Evans and former Tory MP Douglas Carswell.

The party has been in a state of crisis since Nigel Farage stood down as leader after the EU referendum. His replacement, Diane James, lasted just 18 days before quitting, leading to a second leadership election.

Mr Banks had backed Raheem Kassam for the leadership, but he withdrew from the election earlier this week.

The Times reports the millionaire donor has questioned whether Paul Nuttall, who is now widely regarded as the frontrunner, has the “steel” to deal with the problems facing the party.

“There has to be a clear-out of certain people in the party that have caused a hell of a lot of problems,” Mr Banks told BBC Radio 4’s Today programme, adding that Mr Carswell had been the “biggest pain in the backside”.

“I don’t know if it’s over for UKIP,” he said.

Speaking later on Sky News, Mr Banks added that the party was in a “hell of a mess” and did not rule out a return for Nigel Farage if Theresa May messed up Brexit.

“I think at the moment I am not going to deny UKIP’s in a hell of a mess. I think the Labour Party is in a hell of a mess.

“I think that the whole referendum process apart from the Tories, who are naturally ruthless sharks and took advantage of it, the other parties are in a fair degree of trouble.”

He said that Nigel Farage could return to lead the party, but “only if Mrs May flunked Brexit”. While he thinks a comeback is unlikely, he added “you can never tell”.