The former Scottish Conservative leader said the Prime Minister's comments were "pathetic".
It comes after Mr Johnson ordered an investigation into claims Downing Street staff broke lockdown rules by holding a Christmas party last year.
Ms Davidson, who now sits in the House of Lords, tweeted: "None of this is remotely defensible.
"Not having busy, boozy not-parties while others were sticking to the rules, unable to visit ill or dying loved ones.
"Nor flat-out denying things that are easily provable. Not taking the public for fools.
"And today's 'we'll investigate what we've spent a week saying didn't happen and discipline staff for rules we continue to say weren't broken' was pathetic.
"As a Tory, I was brought up to believe in playing with a straight bat. Believe me, colleagues are furious at this, too."
Mr Johnson told MPs he was “furious” about footage apparently showing aides joking about it.
The Prime Minister apologised “unreservedly” for the offence caused by the footage of his then-spokeswoman Allegra Stratton at a mock press conference.
But he insisted he had been repeatedly assured “there was no party and that no Covid rules were broken”.
Speaking at Prime Minister's Questions, Mr Johnson said he had asked Cabinet Secretary Simon Case “to establish all the facts and to report back as soon as possible – and it goes without saying that if those rules were broken then there will be disciplinary action for all those involved”.