Boris Johnson's premiership is past the point of no return and he should resign, a financier who has given the Conservatives more than £3m says.
John Armitage told the BBC that challenges facing the West demanded "serious, engaged" politicians.
He has given the Tories more than £500,000 since Mr Johnson became PM but donated £12,500 last year to Labour.
A Conservative spokesperson said the government remained "fully focused on delivering for the British people".
On Tuesday, Mr Johnson carried out a mini-reshuffle of his cabinet, as he attempts to reconnect with MPs questioning his future following revelations of parties held in Downing Street during Covid-19 lockdowns.
Labour, the Liberal Democrats, the SNP and some of his own MPs are calling on him to resign.
But Mr Johnson has insisted he will change the culture within No 10 and focus on delivering the agenda that won him a landslide in the 2019 election.
Mr Armitage, co-founder of the hedge fund firm Egerton Capital, told the BBC's political editor, Laura Kuenssberg, he found the current political situation "tremendously upsetting".
Describing himself as a "middle-of-the-road conservative", Mr Armitage said he admired politicians on the left and right, and said current global challenges to the West required ''very serious, engaged politicians with a sense of purpose".
"Politicians should go into politics to do good for their country," Mr Armitage added. "That is the overwhelming reason to be in politics. I don't think it's about your own personal sense of getting to the top of a snakes-and-ladders game.
"And I feel that, if you lose moral authority, and if you do things which the average person - your mother, someone you try to explain to, someone who you admire - if you do something or say something, which on the front page of the Sunday Times looks terrible, and you do that consistently, and you betray a sense of not really caring, I think you should leave
"And I find the lack of honour inherent in modern politics, incredibly distressing."
Asked if Mr Johnson was "past the point of no return", he replied: "Well, personally yes."
Mr Armitage has given £3.1m to the Conservatives although he described himself as a "very low-profile person" who does not seek political influence.
He has also given much smaller amounts of money to Labour, made individual donations to the former Labour MP Frank Field, and contributed to the Remain campaign in 2016.
It has been a turbulent few months for Mr Johnson, whose government has been dogged by a series of political controversies.
The turmoil deepened at the end of January following the publication of a highly critical interim report by senior civil servant Sue Gray, who was investigating lockdown parties in Downing Street.
In long-awaited findings, Ms Gray said there were "failures of leadership and judgment" and a culture of "excessive" alcohol consumption within Downing Street.
Since then, several of Mr Johnson's senior advisers have resigned, leading to the appointment of a new communications director and chief of staff.
Tuesday's cabinet reshuffle marked the latest attempt to improve the operation in Downing Street and move on from weeks of political discord within his party.
But expressing concern about the standards of leadership in No 10, Mr Armitage said: "It's about more than like, 'if I'm failing, I've done a few things wrong, oh gosh, I'm going to change my advisers, God above!'.
"What about a sense of personal responsibility? You know, 'I'm going to change my chief of staff and it will all be fine.' Oh, really?"
Mr Armitage said he had told the Conservative Party he would not be giving them any more financial support as things stand, but he does plan to remain a member to retain his vote.
He said there were "competent" members of the current cabinet, but declined to name a potential successor to Mr Johnson.
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