The Conservative Party is taking an interventionist turn under Theresa May's leadership, with the prime minister promising to cap "rip-off" energy prices during a speech in York on Tuesday 9 May.

The Tory premier was forced to defend the move, which party sources claim will benefit around 17 million households across the UK by up to £100 ($129) a year, since her predecessor David Cameron claimed former Labour leader Ed Miliband was in a "Marxist universe" when he proposed a similar policy.

"We are Conservatives. We believe in free markets and competition, but we want to see competition working," May told reporters.

But the manifesto commitment has been criticised by a range of business bodies, including the influential Confederation of British Industry (CBI).

"A major market intervention, such as a price cap, could lead to unintended consequences, for example dampening consumers' desire to find the best deal on the market and hitting investor confidence," said Josh Hardie, the deputy director-general of the CBI.

A 2016 report from the Competition and Markets Authority (CMA) made 30 recommendations for energy firms after finding that customs had been paying £1.4bn more in "excessive prices" because the market was not fully competitive.

May's interventionist tendencies were first outlined during her keynote speech to the 2016 Conservative Party conference in Birmingham, the West Midlands.

"I said that the government I lead will be driven not by the interests of a privileged few, but by the interests of ordinary, working-class families," she said.

"People who have a job, but don't always have job security. People who own their own home, but worry about paying the mortgage. People who can just about manage, but worry about the cost of living and getting their kids into a good school."

May's latest policy announcement comes just a month before the general election on 8 June. The latest opinion poll from Kantar, of more than 1,200 people between 4 and 8 May, gave the Conservatives a 16 point lead over Labour (44% versus 28%).