Mr Sunak, along with the majority of Conservative MPs, opted to stay away from last night’s vote on the Privileges Committee’s findings on Mr Johnson’s conduct.
In the Senedd, Mark Drakeford said: “It was an astonishing act of political cowardice last night that the Prime Minister of this country failed to back the report of an independent committee of the House of Commons charged with that investigation, and coming up with a majority of Conservative MPs on that committee with a series of recommendations.
“The fact that the Prime Minister failed to support the committee I think is an astonishing example of his political weakness and I’m quite sure that he will and he deserves to regret it in future as well.”
He was responding to new Plaid Cymru leader Rhun ap Iorwerth who said that last night’s vote showed that the Conservatives “hold basic principles of integrity and trust in contempt."
“It's incredible, isn't it that a majority of Welsh Conservative MPs effectively sided with Boris Johnson last night by not voting on that Commons report", he said.
Only seven MPs voted against the Privileges Committee's conclusions in the Commons vote. The sanction recommended by the Tory-majority panel, endorsed by 354 votes, will see Mr Johnson lose special access to to Parliament usually granted to former MPs.
Mr Johnson should also have faced a 90-day suspension for misleading the House when he told the Commons that Covid rules were obeyed in No 10 despite parties taking place, the committee also concluded.
However, he evaded this suspension by quitting as an MP after he was told in advance of the committees findings.
Mr Sunak has claimed that he did not want to publicise his view on the conclusions as he “wouldn’t want to influence anyone in advance” of the free vote.
He said he respected the committee, but that it was “a matter for Parliament and members as individuals” rather than Government.
The Prime Minister's official spokesperson said that he was attending meetings on Monday afternoon before a dinner in the evening.
However, his absence has been criticised by other parties. A Labour Party spokesperson said: “Rishi Sunak is too weak to lead a party too divided to govern and working people are paying the price.
“Rather than distance himself from his disgraced predecessor, Rishi Sunak has repeatedly demonstrated he’s just more of the same Conservative failure we’ve had to put up with for 13 years now. It’s time for a change.”