Conservative Party chairman Oliver Dowden has resigned after the party slumped to two by-election defeats.
Mr Dowden said in a letter to Boris Johnson that the defeats were "the latest in a run of very poor results" and added: "We cannot carry on with business as usual."
He is the first Cabinet minister to fall on his sword in the wake of the pressure swirling around the prime minister over the partygate scandal - which has already prompted 148 Tory MPs to oppose the PM in a vote of no confidence.
The Conservatives saw a majority of 24,000, or more than 40%, evaporate in Honiton and Tiverton, where they were defeated by the Liberal Democrats - a record reversal for the party.
They also lost Wakefield, the Red Wall seat snatched by the Tories in 2019, which went back to Labour.
Mr Dowden said: "Our supporters are distressed and disappointed by recent events, and I share their feelings.
"We cannot carry on with business as usual.
"Somebody must take responsibility and I have concluded that, in these circumstances, it would not be right for me to remain in office."