Theresa May's joint chiefs of staff Nick Timothy and Fiona Hill have both resigned.
The top aides both stepped down after facing furious criticism from party MPs and officials who said they played a significant role in the party's poor performance at the General Election.
Mrs May had been given an ultimatum from some MPs warning her to sack the pair by Monday or face a leadership contest.
Mr Timothy announced his resignation in a blog post saying: "I take responsibility for my part in this election campaign".
He acknowledged that one of his regrets was the way Mrs May's social care policy - dubbed the "dementia tax" - by critics had been handled.
However, he rejected "bizarre media reports" that he had personally overseen the bungled social care manifesto commitment in the post on Conservative Home.
Shortly after he announced his resignation, a Downing Street spokesman confirmed that Ms Hill had also stepped down.
In a statement, she said: "It's been a pleasure to serve in government, and a pleasure to work with such an excellent Prime Minister.
"I have no doubt at all that Theresa May will continue to serve and work hard as Prime Minister - and do it brilliantly."
Both aides had faced bitter anger from Conservative figures who described them as "poison" and blamed them for creating a "toxic" environment within Downing Street.
Critics within the party believe that Mrs May and her advisers declined to take enough outside advice on the election campaign and misread the mood of the country.
The Prime Minister's former spin doctor also accused the pair of top aides of "damaging" aggression and arrogance in an article describing her painful" ten months working at Downing Street.
Their resignation leaves Mrs May looking increasingly isolated.
Shadow health secretary Jonathan Ashworth, who played a frontline role for Labour in the election campaign, said the Tories were in an "utter mess".
He tweeted: "Top Theresa May advisor resigns. More chaos in the Tory Party, more weak & unstable govt. What an utter mess from this shower."