Zac Goldsmith’s sister Jemima has criticised how his campaign for London mayor was run, while former Tory Cabinet minister Baroness Warsi called it an “appalling dog whistle campaign”.
Tactics used in the fight for the post were branded a mistake by a senior Tory, while others have questioned the impact on the party.
She also congratulated Sadiq Khan, who is set to become London’s first Muslim mayor, calling him a “great example to young Muslims”.
Their brother Ben Goldsmith later took to Twitter to defend him, claiming: “Labour planned to paint Zac as a racist from the start. The questions asked of Sadiq were legitimate.”
Baroness Warsi tweeted: “Our appalling dog whistle campaign for #LondonMayor2016 lost us the election, our reputation & credibility on issues of race and religion.”
The Conservative mayoral candidate and Prime Minister David Cameron sought to link Mr Khan with Muslim “extremists” in the bitter race to succeed Boris Johnson in City Hall.
Andrew Boff, the Conservative group leader on the Greater London Assembly, launched a devastating attack on the way Mr Goldsmith’s campaign was run.
Mr Boff said the campaign had “done real damage” and had “blown up” bridges the Conservative Party had built with London’s Muslim communities.
“I mentioned that I thought this was a mistake for future integration in London. If you are a London politician this is just a bizarre thing to do,” he said.
He told the BBC’s Newsnight programme it was an error to “equate people of conservative religious views with sympathising with terrorism” but said it was not an example of so-called dog whistle politics because it was such a prominent part of the campaign.
“I don’t think it was dog whistle because you can’t hear a dog whistle – everybody could hear this,” he said.
“It was effectively saying that people of conservative religious views are not to be trusted and you shouldn’t share a platform with them. That’s outrageous.”
Mr Boff said the Tories had done well where they had actively engaged with the Muslim community in the borough of Newham, but “now those bridges that have been built, a few of them have been blown up by this campaign”.
He said there were policy areas where the Tories could have challenged Mr Khan – but instead they focused on the extremist issue.
“It was ridiculous,” said Mr Boff, who ran against Mr Goldsmith for the Tory nomination.
“I do believe it’s going to affect Conservatives at the sharp end, especially in those parts of London where there is a high Muslim population.”
Political strategist Sir Lynton Crosby ran Mr Goldsmith’s campaign after guiding Mr Cameron and the Conservatives to general election victory last year.