Lord Frost hit back at Theresa May after she blamed Boris Johnson and Liz Truss for the Tories' woes.
The former Brexit minister said to take what the ex-prime minister says about politics "with a very large pinch of salt".
He pointed to Mrs May's "bungled" Brexit negotiations as well as general and European election results under her premiership.
The Tory peer wrote on X: "Theresa May is a former leader of our party & should be treated with respect.
"But I can't forget her disastrously bungled Brexit negotiations, her near-loss of an election to the worst Labour leader ever, and her delivery of a Conservative vote share of 9% in the 2019 Euro 'elections'.
"Boris Johnson successfully cleared all this up. So take what she says about how to do politics, and who is and is not Conservative, with a very large pinch of salt."
Lord Frost's intervention comes after Mrs May accused her successors Mr Johnson and Ms Truss of destroying the Conservative Party’s popularity.
Asked why the Conservatives were so unpopular, she told a press gallery lunch in Westminster: “Obviously we’ve been in for 14 years, and it’s always harder for a party that’s been in government for a significant period of time.
“Let’s face it, with Boris the lockdown parties exacerbated a sense, that was already there from people, of it was one rule for MPs and another rule for them.
"And then with Liz, what happened with the markets, that just disrupted the sense that this was a party of economic management."
Mrs May said that voters had seen “quite a bit of change under us and things happening. They wanted some stability, and I think Rishi has brought that stability".
She also rejected the suggestion that the result of the upcoming general election was a foregone conclusion
Mrs May said Sir Keir Starmer lacked the popularity of Sir Tony Blair and Labour faced a tough task in winning the number of seats it needs even for a majority of one.
She insisted the UK needs to remain in the European Convention on Human Rights (ECHR) amid growing Tory calls to quit.
In a further rebuke to the right of her party, Mrs May rejected the suggestion that Reform UK leader Richard Tice and honorary president Nigel Farage should be welcomed into the Conservative Party.