Mr Baron became the second Tory MP to call for Mr Johnson's resignation after the release of Sue Gray's much-anticipated report on Wednesday.
His announcement came after Julian Sturdy MP claimed he could no longer give the PM "the benefit of the doubt".
Minutes after Mr Baron, David Simmonds MP became the third Tory to twist the knife.
He said that while the Government enjoys public confidence, "the Prime Minister does not".
Ms Gray's report concluded that many of the events investigated "should not have been allowed to happen", some of which were attended by "leaders in Government".
It has piled pressure on Mr Johnson who said he takes "ultimate responsibility" and that he is "appalled" by the behaviour highlighted.
++ SUE GRAY REPORT: FOR A FULL RUNDOWN OF WHAT WE KNOW SO FAR, CLICK HERE ++
John Baron and David Simmonds
UK Parliament
Boris Johnson delivered a statement to the House of Commons following the publication of Sue Gray's report yesterday
House of Commons
In a statement published on his website on Thursday, Mr Baron said: “The Sue Gray report and the Metropolitan Police investigations paint a shameful pattern of misbehaviour during the pandemic as the rest of us kept to the Covid regulations.
"Those responsible for setting the rules have a special duty to adhere to them.
“However, for me the most serious charge against the Prime Minister is that of knowingly misleading Parliament.
"Given the scale of rule-breaking in No 10, I cannot accept that the Prime Minister was unaware.
"Therefore, his repeated assurances in Parliament that there was no rule-breaking is simply not credible.
“For some, this may appear a trivial point given world events.
Yet a bedrock principle of our constitution is that we can trust the responses we receive in Parliament to be truthful and accurate.
"Parliament is the beating heart of our nation. To knowingly mislead it cannot be tolerated, no matter the issue.
"Whether or not the Prime Minister is an asset to the party or the country is of less importance.
“Having always said I would consider all the available evidence before deciding, I’m afraid the Prime Minister no longer enjoys my support – I can no longer give him the benefit of the doubt.”
