Sir Keir Starmer called Boris Johnson a 'bulls***er' when asked for a nickname for the Prime Minister.
The Labour leader also said it is "nonsense" to claim his party has not been leading on the cost-of-living crisis. He was interviewed by Iain Dale and Jacqui Smith at an Edinburgh Fringe event on Friday.
Mr Dale said there had been criticism of Labour's stance but said "all politicians have to take holidays".
Sir Keir said: "This business that we haven't been leading on this is pretty nonsense actually."
He continued: "The Labour party actually has been all over this for the best part of a year because energy prices and what we would do about them was the dominant theme of our conference last year. I said we've got to have not just crisis management, but deal with the problem more substantially."
He said he was a "massive fan" of insulating homes better.
Mr Dale also asked the Labour leader to come up with a nickname for the Prime Minister, who has in the past taunted him as "Captain Hindsight" and "Captain Crasheroonie Snoozefest".
He said: "What frustrates me is that if you're the Prime Minister, you're in the unique position of pulling those levers to make a real change for people. That man Johnson made promises he never intended to keep.
"They were empty promises. That is the worst kind of politics, pretending you're going to do something - knowing you're not going to do it and not caring.
"Levelling up, there isn't a strategic plan or the resources to do it. Dealing with entrenched inequality is hugely important."
He concluded by saying: "He's a bulls***er."
The Labour leader also spoke about his own "modest" upbringing and how it affected his understanding of the pressures people are facing today.
He said: "I'm not complaining. I was very, very lucky.
"I went from that working-class background to heading up the Crown Prosecution Service. I had a fantastic opportunity.
"But we couldn't pay our bills sometimes, we struggled."
He also praised his deputy leader Angela Rayner, saying he would make her deputy prime minister if Labour won the next election.
He said: "I think Angela's story is absolutely incredible and she's an incredible individual."
Don't miss the latest news from around Scotland and beyond - sign up to our daily newsletter here.
