Bendor Grosvenor, who is a co-presenter of BBC4’s Britain’s Lost Masterpieces, has voted Conservative for more than 20 years but blames “Johnsonian Toryism” and Brexit for driving “the broad church into the crypt”.

Grosvenor, 41, said that scandals, political purges and hardline support for leaving the European Union was driving many old-fashioned, liberal conservatives away.

The broadcaster, who was educated at Harrow and Cambridge and lives near Edinburgh, says he will now vote for the SNP.

He became a Tory member in 1997 and took an active role in the party after meeting Hugo Swire, the former Conservative shadow arts minister. “We ended up writing the cultural and heritage section of the Tory manifesto for the 2005 election. That was the highlight of my political career,” he said.

“Each time Kenneth Clarke ran for the leadership I was there supporting his campaign as a donor and a bit of a worker.”

Recent political events, including the illegal prorogation of parliament in August, had filled him with frustration and sadness, he said. “The Tory party has been the most successful political party in modern history, and yet it is running its campaign at the moment as if it couldn’t run a bath.”

Comments made this week by the Conservative MPs Jacob Rees-Mogg and Andrew Bridgen about the Grenfell fire were the last straw for Grosvenor, who is related to the Duke of Westminster.

Mr Rees-Mogg, the leader of the Commons, said that it would have been “common sense” for tower residents to ignore advice given to them by firefighters and flee the burning west London tower block in 2017. After public outcry Mr Bridgen went on Radio 4’s PM to defend the comments, saying that Mr Rees-Mogg would have made a “better decision” than authority figures on the night.

On Twitter Grosvenor wrote: “I’ve voted for, worked for, been a member of, and even a historian of, the Conservative Party. It is time now to destroy it, to smash it utterly, so that people like Rees-Mogg and Bridgen can never be near power again.

“It was that jaw dropping interview with Andrew Bridgen on Radio 4 after Jacob Rees-Mogg’s similarly extraordinary comments. I have been drifting apart from the conservatives for a long time. But yesterday I just thought it is time.”

The expulsion of Tory ministers and MPs who rebelled against Boris Johnson’s Brexit plan in September — including Mr Clarke, who has now stood down — was another “watershed moment where people like me thought there was no return,” Grosvenor added. “I plead guilty to what Brexiteers call Brexit derangement syndrome.

“Small ‘c’ conservative values I have always signed up for — as old-fashioned as it sounds, valuing the constitution as it is in part. On the decision to prorogue parliament and the all the legal ramifications and Supreme Court case: If the Conservative party has contempt for those basic conservative tenets then what is left of it?”

The Tory leadership was also in the crosshairs. “When I was working in politics I encountered Boris Johnson a few times,” Grosvenor said. “He always seemed to be shambolic and to bear the impression of whoever sat on him last. I don’t think that proves to make effective leadership at moments of crisis like this.”

Grosvenor, who recently discovered a work by Peter Paul Rubens for his TV show, now pins his hopes on the SNP before the general election on December 12.

“I think I would vote for the SNP. Brexit, the May government and now the Johnson government have so shaken the foundations of everything that we took to be the settled constitutional and political order. Now all bets are off.

“I don’t view any of the options with great enthusiasm. It’s a moment of profound frustration and sadness that it’s come to this.”