THERESA May has denied that her plans for a cap on "rip-off" energy prices are a repeat of an old Labour policy derided by David Cameron as "Marxist".

The Prime Minister said 17 million households would benefit by up to £100 from the cap on poor value standard variable tariffs.

But Labour said there was no guarantee that the cap would stop energy prices going up next year, while energy companies said the move would distort the market and undermine moves to improve choice.

Speaking to Conservative General Election candidates in York, Mrs May said that last year's investigation by the Competition and Markets Authority had shown consumers were spending £1.4 billion a year more than they would in a truly competitive market.

"It's right, as does everyone sitting around the Cabinet table, for Government to take action to support working families," declared the Prime Minister.

"What we are talking about is a cap on energy prices that will be set by an independent regulator and that will be a reflection of the market. Crucially, it will be possible for that cap to move, and the independent regulator will set it.

"The key thing is that people are clearly paying too much for their energy bills today. Too many ordinary working families, too many vulnerable people, find themselves on tariffs that are above what they should be paying, and that's why we are taking action."

Mrs May insisted her plan was different from the energy freeze proposed by Labour in 2015, which led Mr Cameron to accuse the party's then leader Ed Miliband of wanting to live in a "Marxist universe".

Asked if she was herself now living in a Marxist universe, she replied: "No. First of all, we are Conservatives. We believe in free markets and competition but we want to see competition working...

"Ed Miliband didn't suggest a cap on energy prices. Ed Miliband suggested a freeze on energy prices which would have frozen them so that people paying above the odds would have continued paying above the odds, and crucially the prices would not have gone down. Under our cap prices will be able to come down," she added.