When asked whether the Prime Minister should resign after the war, the Scottish Conservative leader, who has now changed his position three times on Mr Johnson’s leadership, said: “Yes.”
Mr Ross told Sky News: "Yes, then I think he should step down, because I've said it is the stability in the UK Government at this time of conflict."
He added: "To directly answer your question, as I'd tried to do, is yes."
Mr Ross told journalists the Prime Minister did not have his “unqualified support”, but said "it is not the time” for a vote of no confidence following the publication of the Sue Gray report.
In the report published on Wednesday, senior civil servant Sue Gray stated many of the events "should not have been allowed to happen" and that staff who raised concerns were not treated with respect.
Senior leadership at Downing Street "must bear responsibility for this culture", she added.
Descriptions of parties during lockdown in Westminster outlined in the report include excessive drinking, wine spilled on walls, staff being sick and having an "altercation".
Earlier, Mr Ross said: "I changed my position because of the war in Ukraine, therefore if the war in Ukraine is over then the Prime Minister doesn't have the same support that he has at the moment – not just from me, but from other MPs who previously called for him to resign and go."
After seeing certain pictures included in the report before it was published, Mr Ross previously said the Prime Minister must outline why he believed this behaviour was acceptable.
He added: “To most, these pictures seem unjustifiable and wrong.”
A recent YouGov snap poll of more than 2,000 adults in the UK found a larger proportion (72 per cent) of Scots think Mr Johnson should resign than Britain-wide (59 per cent).
The SNP said Mr Ross must resubmit his letter of no confidence to the 1922 backbench committee or risk losing any remaining credibility.
SNP MSP Graeme Dey said Mr Ross was “out of excuses” for not calling on the Prime Minister’s resignation following the report’s publication.
Conservative MSPs, including Finlay Carson, Alexander Stewart, Maurice Golden and Brian Whittle, have all called for the PM to resign.
In an interview with Representing Border, Mr Carson said if the Gray report had been about him, “I would be considering my position”.
However, Scottish Secretary Alister Jack has given Mr Johnson his “full support”.
Mr Jack said the PM has taken “full responsibility” and was now “tackling the cost of living at home and leading the international response to Russia’s invasion of Ukraine”.
SNP Westminster group leader Ian Blackford said Mr Johnson had engaged in a culture of “drinking and debauchery” as he questioned the PM in the Commons.
Mr Blackford said: “Empty bottles littering offices, rooms so crowded people were sitting on each other’s laps and security forced to intervene because the parties were so outrageous.
"At the centre was the Prime Minister orchestrating it, grabbing a glass for himself in order to toast the party goers.
"For eight months, we’ve heard every excuse under the sun, but now we’ve all seen the damning photo evidence. While people stay at home to protect the NHS, the Prime Minister was engaging in drinking and debauchery that makes a mockery of the sacrifices each and every person made.”
Scottish Labour leader Anas Sarwar claimed Mr Ross failing to call for the Prime Minister’s resignation meant his claim to be the “defender of the United Kingdom is blown out the water”.
Mr Sarwar said: “This is a devastating report from Sue Gray.
“It sets out clearly that while people across our country struggled with loneliness and despair, our Prime Minister presided over parties until 4am and the mistreatment of cleansing and security workers.
“Boris Johnson has shown a total disrespect for his own staff and for the British public.
“If there is to be any shred of decency, morality and dignity back in our politics, then Boris Johnson must resign. And if the Conservative Party had any integrity whatsoever they would kick him out.
“If they fail to do so it is on us, the British people, to unite across the country to boot him out of office.
“This puts pressure on Douglas Ross, who must re-submit his letter to the 1922 if he wants to claw back any credibility or principles.”
He added: “He needs to grow a backbone and boot out Boris Johnson.”
Scottish Liberal Democrat leader Alex Cole-Hamilton said: "This report lays out in black and white the culture of rule-breaking and lawbreaking presided over by the Prime Minister.
"He made the laws, broke them and then lied about it to the British public.
"This is also a defining moment for Douglas Ross in his leadership of the Scottish Conservatives. I have spoken to members of his party and Conservative voters who are shocked that he is still standing by a Prime Minister who is only out for himself. He needs to grow a backbone and submit his letter of no confidence.
"Every household in Britain faces eye-watering costs at the petrol pumps, sky-rocketing food prices and massive energy bills. It needs new hope and new leadership."