Ed Miliband's promise to freeze energy prices is a "Wonga-like" offer from a man with no interest in energy efficiency, according to Boris Johnson.
Comparing the Labour leader to "a turbine on a windless day", the Mayor of London said Miliband "wasn't remotely interested" in detailed plans to improve domestic energy efficiency in the English capital when he held the Energy Secretary brief - and was more interested in trading jokes about the forthcoming elections.
In his weekly column for The Daily Telegraph, he wrote: "I don't think I have ever told you about my last official meeting with Ed Miliband. I must have somehow blanked it out, as one of those experiences that is just too harrowing to relate.
"It took place a few years ago, and my City Hall team was very excited in the run-up. We had an absolute corker of a plan, you see. We had the spreadsheets, the data, the options - and all we really needed was for Government to get behind it, and make sure that London got its fair share of the funding."
Johnson said City Hall wanted to launch a huge drive to improve energy efficiency and help cut heating costs.
He added: "I was interested in the plan as a way of helping the planet AND helping people in tough times. As for Ed - well, it was frankly a bit disheartening. He wasn't remotely interested. He didn't want to talk about retrofitting, and as I gabbled away about a new legion of "boiler bunnies" bouncing up to your door, I was aware that a deep tranquillity had settled on the minister.
"He didn't want to talk about cutting the cost of living. He just wanted to trade jokes about the forthcoming General Election; and as one of my team put it later, 'he was only vaguely in command of his brief and had no interest in achieving anything'.
"We wrote a long and optimistic follow-up letter, hoping that perhaps he had been taking it in. Nada. Not a peep. So now that he is in opposition, and struggling with his ratings, I find it frankly incredible that he can seriously pretend to want to do something for the hard-pressed energy consumers in this country, and I find it astounding that so many people are falling for his Wonga-like offer."