The Scottish secretary - who is also the MP for Dumfries and Galloway - defended his support for the under-fire Prime Minister when challenged in the House of Commons about Downing Street party allegations.
Labour shadow Scottish secretary Ian Murray said: "The Sue Gray report released on Monday was utterly damning about the Prime Minister's conduct yet the Secretary of State continues to back him against the wishes of his own Scottish Conservative leader (Douglas Ross) - who I noticed is not in the chamber for Scottish questions today.
"We now know the Metropolitan Police is investigating no less than 12 incidents in Downing Street with more allegations every day.
"It's little wonder then that a recent poll found that the Prime Minister was as unpopular as Alex Salmond in Scotland.
"Does the Secretary of State think that the Prime Minister refusing to do the decent thing and resign is good for the union or helps those who want to break it up?"
Mr Jack replied: "The Prime Minister is resolute in opposing a second Scottish independence reference and, therefore, very good for the union."
Mr Jack earlier told MPs: "February 2 is Groundhog Day - of course in Scotland every day in Scotland feels like Groundhog Day with the SNP's incessant calls for another independence referendum."
Mhairi Black, the SNP's Scotland spokesperson, said the "strength of any union rests upon the confidence that people can have in those running things".
She added: "I know I disagree with the minister's political judgment, so let me appeal to his business judgment here.
"Hypothetically, if he was handed evidence that the man running his company had been incompetent, dishonest and were subject to a police investigation, bringing the entire company into disrepute, would he let them carry on in the role or would he expect them to step back?"
Mr Jack replied: "As it has been said at this despatch box many times, the Prime Minister is very sorry for what happened. He has apologised.
"He said he could have done things differently, with hindsight he would have done, but it is also the case no-one has said he is subject to a police investigation."
In response to separate questions, Mr Jack insisted Mr Ross has his "full backing".
Mr Jack said: "Douglas Ross absolutely is the leader of the Scottish Conservatives, he is put there by the membership because we're a constitutionally-devolved organisation, he's doing a very good job, he holds Nicola Sturgeon to account and he has my full backing."