But Sir Keir said: “There’s a big difference between the forensic lawyer and Keir on the football pitch. If I was Keir on the football pitch, I think I’d be chucked out by the Speaker.
“So we’ve got to be careful about how far we go.”
He said: “Let me get out there, let me take the mask off, because we’ve been living in restrictions.
“As we come out of this, this allows the space to open up, the pandemic allows the political space to open up, the restrictions allow me to open up.”
Asked what he would say to Boris Johnson right now, he said: “Move over, we’re coming.”
But asked what he would say to the Prime Minister on the football pitch, he said: “I’d probably knock him over.”
On Labour’s prospects, Sir Keir said: “I’m not going to pretend the last few weeks have been easy, but there’s a huge emotion that runs through the Labour Party, and we lost in Hartlepool, we lost badly.
“We had to make changes, so on things like anti-Semitism, it was really important to me and to the party, I think, to the country, that we dealt with anti-Semitism,” he said.
“We’ve begun to do that, taken some really, really important steps. We’re turning the party around.”
During the interview Sir Keir refused to say whether he had ever taken drugs, despite Mr Morgan asking him several times.
“I spent my university days in the library,” he said.