Keir Starmer branded Boris Johnson a "bulls****er" in an extraordinary tirade - as he rubbished claims he had not been leading on the cost-of-living crisis.
Speaking at the Edinburgh Fringe Festival, he said the Labour Party had been "all over" the issue for a year.
He claimed Mr Johnson had made "empty" promises he never intended to keep, telling interviewers Iain Dale and Jacqui Smith: "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."
Defending his response to the unfolding energy crisis, Mr Starmer 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."
Turning his ire on the outgoing Prime Minister, Mr Starmer 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."
Earlier today the opposition leader said he will outline his party's plan for tackling the energy price crisis on Monday.
Mr Starmer told reporters: "We do need a strategic, a credible, plan and that's exactly what's missing from this Government.
"For weeks now, and for weeks to come, we've seen the spectacle of two leadership hopefuls fighting each other, internal battle, actually arguing about just how bad they've been in Government and a Prime Minister who is a lame duck because he recognises there's a problem with energy bills, but says I'm not going to do anything about it.
"So, yes, we need a strategic plan, and that's what I will be setting out on Monday."
The Labour leader has come under fire for not being more visible as the crisis deepens.
But shadow business secretary Jonathan Reynolds defended his boss, telling LBC: "He is working like he does every day on the job.
"I honestly cannot fault the man's work ethic." Mr Reynolds added: "I think he is spending some time with his family and that is the right thing to do, but I cannot fault the man's work ethic, he has been relentless since he became leader of the opposition."
