Zac Goldsmith has "blown up bridges" by linking Labour's Sadiq Khan with Muslim "extremists", a fellow Tory has said.
Andrew Boff, who Mr Goldsmith defeated in the Tory mayoral candidate contest, said it was an error to "equate people of conservative religious views with sympathising with terrorism".
Mr Boff, who is seeking re-election to the London Assembly, told the BBC: "If you are a London politician this is just a bizarre thing to do."
Mr Goldsmith's team is yet to comment.
Both Mr Goldsmith and Prime Minister David Cameron sought to link Mr Khan with Muslim "extremists" by saying he had shared platforms with people who had extremist views.
Mr Boff, the former Conservative group leader on the London Assembly, told the BBC's Newsnight programme: "I mentioned that I thought this was a mistake for future integration in London."
He said Mr Goldsmith's approach was not an example of so-called dog whistle politics, adding: "I don't think it was dog whistle because you can't hear a dog whistle - everybody could hear this.
"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 Khan has previously said he had "never hidden" the fact that, as a former chairman of Liberty and a human rights lawyer, he had acted for "some pretty unsavoury characters".
Mr Boff said the party had actively engaged with the Muslim community in Newham, but "now those bridges that have been built, a few of them have been blown up by this campaign".
Baroness Kramer, who lost her Richmond parliamentary seat to Mr Goldsmith in 2010, was also critical of the Conservative campaign.
Speaking to BBC Radio London, the Liberal Democrat peer said: "Some of the language has felt vindictive."
She said "innuendo" used about religion and race had been divisive.
The result of the mayoral election is due on Friday evening, with Mr Khan ahead on first preference votes.