Boris Johnson has lost the support of Hereford and South Herefordshire MP Jesse Norman.
Former minister Jesse Norman said he had supported Boris Johnson for 15 years but no longer could.
Mr Noman tweeted: “I have supported Boris Johnson for 15 years, for the London Mayoralty and for PM.
“Very sadly, I have written to him to say I can no longer do so.”
He signalled that he had written a letter of no confidence in the PM to Sir Graham Brady, chairman of the 1922 committee of backbench Tory MPs.
If Sir Graham gets 54 such letters, it would trigger a vote in Mr Johnson’s leadership.