Britain's leading Conservative Party lost two seats in the by-election held Thursday, according to the preliminary results Friday morning.
The elections held for the seats for Wakefield, and Tiverton and Honiton were won by Labour and Liberal Democrat candidates, respectively, after a poor performance from the Tories.
Oliver Dowden, the party chairman, resigned following the election defeat in two constituencies held by his party previously.
Dowden said "the latest in a run of very poor results for our party" made him resign in a letter to Prime Minister Boris Johnson.
"Our supporters are distressed and disappointed by recent events, and I share their feelings," he said.
"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."
"Somebody must take responsibility," Dowden also said in his letter.
The poor result for the Conservative Party is expected to increase pressure on Johnson who survived a no-confidence vote earlier in June.
He got the support of 211 Tory MPs, with 148 voting against him in a ballot triggered by an announcement from the 1922 Committee -- the Conservatives' parliamentary group in the House of Commons.
According to party rules, such a vote cannot be triggered for a year.
Johnson is in Rwanda for a Commonwealth leaders summit and he is expected to go to the G7 summit in Germany and then to Madrid for a NATO summit.
For months, Johnson has faced withering criticism over parties at No. 10 Downing Street during the COVID-19 lockdown, and the party's poor performance in the May 5 local elections also raised eyebrows, triggering speculation that it may lose the next general election under his leadership.