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".
Before the final declaration on Friday, the Conservative candidate looked set to lose the City Hall race to Labour's Sadiq Khan.
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 Mr Khan, set to be London's first Muslim mayor, calling him a "great example to young Muslims".
Baroness Warsi tweeted:
The Conservative mayoral candidate and Prime Minister David Cameron sought to link Sadiq Khan with Muslim "extremists" in the bitter race to succeed Boris Johnson in City Hall.