Ian Blackford was keen to defend comments he made in the House of Commons as he appeared on Good Morning Britain on Tuesday.
The SNP Westminster leader was thrown out of the Commons on Monday after claiming Prime Minister Boris Johnson had 'wilfully misled parliament'.
In a heated statement after the Prime Minister said he was "sorry", Mr Blackford told the Commons: "This Prime Minister has lied and misled the House."
As he appeared on the ITV breakfast show, Mr Blackford defended his comments as he spoke to Susanna and her co-host Ed Balls.
“They are certainly not words I would use lightly, but the fact remains that this is a Prime Minister who has let down everybody in these islands with his behaviour over Covid," Mr Blackford said as the hosts referenced why he was told to leave the Commons.
He continued: "What we have is a Prime Minister who has repeatedly broken his own rules and we now know, off the back of the Sue Gray report, that there are 12 different events at Downing Street that have been referred to the Metropolitan Police - as many as four that the Prime Minister has been believed to be at.
"Crucially in all of this, the Prime Minister came to the House of Commons on the 8th December [2021] and denied that these parties had taken place.
"Not only did he deny that the parties had taken place, he gave no indication that he had been at those parties. This is a serious matter for the House of Commons, because he has misled the house and on the basis of having misled the House and lied, that in itself is a resignation matter."
Making it clear that he feels Mr Johnson should resign, he continued: "In public life there has to be dignity and self respect. If you breach the trust within you, you do the honourable thing and resign.
"If I had withdrawn what I'd said yesterday in the House of Commons, I would have been guilty of doing what the Prime Minister has done and that would have been lying to everybody watching.
"One of these days, the Prime Minister is gonna have to accept that he has abused the trust that was put in him when he became Prime Minister, he should have gone by now.
"And this morning, my message to Tory MPs, they're going to have to do the job because quite simply, this man is not fit for purpose, he's not fit to be Prime Minister."
Good Morning Britain airs weekdays at 6am on ITV and ITV Hub.