Conservative Party Chairman Oliver Dowden resigned after his party suffered two by-election defeats.
In his resignation letter to Prime Minister Boris Johnson, Dowden explained that "someone must take responsibility."
"Our supporters are distressed and disappointed by recent events and I share their feelings. We cannot carry on with business as usual," Dowden wrote. "[...] I have concluded that, in these circumstances, it would not be right for me to remain in office."
The Tories have lost their 24,000 majority in Tiverton and Honiton to Liberal Democrats, while the Labour reclaimed the Wakefield constituency, securing 47.9% of the vote.
Before the Friday defeat, the southwest England seat had been held by the Conservatives for well over a century.
Shortly before the Conservatives' two losses, Boris Johnson was asked by reporters whether he had any plans to resign should his party fail to win the by-elections. The prime minister said the idea was "crazy", noting that governing parties "generally do not win by-elections, particularly not in midterm."
The Tories' by-election misfortunes, now fueled by Dowden's resignation, might increase pressure on Johnson, who has been facing calls to resign himself in the wake of the "partygate" scandal.
Accused of not addressing the coronavirus lockdown parties at Downing Street properly and suspected of lying to Parliament about his involvement in them, the embattled prime minister faced a no-confidence vote that he narrowly survived. With 211 fellow Conservative MPs supporting him, 148 Tories (some 41%) voted to remove him from his position.
However, even Johnson's no-confidence vote victory does not guarantee that he stays in power, with some observes recalling the Theresa May scenario, when Johnson's predecessor was put into a similar position in 2018. Despite surviving the no-confidence vote, she resigned months later after being pressured by her own party - something that might also happen to the incumbent PM.