While the Prime Minister did phone Khan on Sunday to offer his congratulations, it took him until six days after the election to do so publicly.
Despite Cameron's long period of silence public silence, nine other celebrities and public figures managed to find the time to congratulate Mr Khan on social media.
1. William Shatner
2. Hillary Clinton
3. His rival, Zac Goldsmith
4. His rival's sister, Jemima Goldsmith
5. The Daily Mail
6. Conservative minister Sajid Javid
7. New York Mayor Bill De Blasio
8. Ahok Basuki Purnama, the governor of Jakarta
9. Parisian Mayor Anne Hidalgo
Translation: "Congratulations to
@SadiqKhan, elected Mayor of London! Convinced that his humanism & his progressivism will benefit the Londoners!"
Cameron publicly criticised Khan repeatedly in the run up to last week's mayoral election.
The PM said at Prime Minister's Questions that he was “concerned about Labour’s candidate as mayor of London who has appeared again and again and again” on a platform with people he described as extremists.
The Tory candidate Zac Goldsmith said in April that he believed Khan had given “platform, oxygen and cover to extremists”.
But the campaign tactics did not wash well with the Chairman of the Conservative Muslim Forum, who claimed that attempts to link Khan to extremists had increased the risk of an attack by Isis on the streets of Britain.
Mohammed Amin told how, despite being a long-standing Conservative supporter, he was troubled by the Richmond Park MP’s campaign.