Labour's most generous donors are to withdraw financial support worth millions of pounds and have called for the arrest of the party's errant MPs in the wake of disastrous revelations about parliamentary expenses.

The disclosure is a further body blow to Gordon Brown's advisers after 10 days of damaging disclosures. Yesterday a second Labour backbencher, David Chaytor, was suspended from the party whip after admitting he claimed £13,000 for a mortgage he had already paid off, while a senior minister predicted that sitting MPs would be deselected in a purge of parliament.

A poll published today on the website PoliticsHome.com, and previewed exclusively by the Observer, shows that the scandal has driven more than a quarter of voters to change the party they support. The main beneficiary is Ukip, followed by the British National Party and the Greens.

The Observer has spoken to 10 prominent donors and only one pledged to maintain financial support for the already heavily indebted party. Anthony Bailey, the public relations "fixer" who introduced the £1m donor Mahmoud Khayami and has given more than £72,000 himself, said that he would not be donating to Labour and would not encourage others to do so. "The expenses stories are shameful. Anyone who is thought to have broken the law should be investigated by the police immediately," he said. "I have no intention of giving a single penny to Labour at this time, and I cannot see how I could introduce any of my contacts either."

Bailey, who claims to have raised £1.4m for the party over the past two years, added: "I can see many other donors withdrawing their support."

One of Labour's most generous donors, who did not want to be named, but gave more than £500,000 under Tony Blair, said he plans not to give again. "This week comes after a very bad year [for Labour], and that ridiculous mess over the 50% tax band. Gordon Brown has not had much luck, but won't receive a donation from me."

Moni Varma, the millionaire rice importer who has given nearly £30,000, said he will no longer contribute. "If this happened in business or any other walk of life it would lead to prosecution," he said. Asked if he was considering withdrawing support, he said: "Yes, I am disillusioned. They are going downhill. They were a good party for Britain, but now I don't know."

Sir Gulam Noon, the food tycoon who has donated more than £530,000 and was interviewed by police under caution before being cleared during the "cash for honours" affair, said that he was undecided about whether to donate again. "I was interviewed by police for lending money, so those who take it from the public purse should certainly be interviewed. There has been some outrageous behaviour from members of the Labour party," he said.

Only Lord Paul, the industrialist and close friend of Gordon Brown, pledged to remain a financial supporter - but even he admitted that "all hell has broken loose".

With up to a dozen MPs now fighting for their political lives, a senior minister said some were clearly "irretrievable". "Public anger is so overwhelming that MPs even in safe seats are going to have to account for themselves to their local parties and to their public.

"Constituency parties will be looking at this and wondering whether they can win an election if their MP has been revealed to be behaving badly," he told the Observer. "If they come to the conclusion they can't [win the election] there will be MPs who will be deselected, no matter how senior."

He spoke as the chairman of the Luton South Labour constituency association said it had asked its MP, Margaret Moran, to explain herself. She billed the taxpayer for dry rot treatment at a house in Southampton. The chairman, Mahmood Hussain, said: "In our judgment this is something which is very questionable."

Labour's ruling National Executive Committee will discuss plans this week for an emergency mechanism to deselect sitting MPs.

The Tory frontbencher Andrew MacKay, meanwhile faces an emergency meeting in his Bracknell constituency this week. James Gray, who sought to claim the cost of Remembrance Day wreaths, is also considered vulnerable.

The PoliticsHome poll found that of the 27% of voters now prepared to defect from their current party, 28% will back Ukip, 16% the BNP and 15% the Greens. More than eight in 10 voters wanted MPs caught abusing expenses deselected. The poll showed 57% of voters think David Cameron handled the scandal best, compared with 11% saying Brown had.

A YouGov poll for the Greens is set to reveal this week that up to 34% of Britons would now consider voting Green.