The Prime Minister appeared to surprise his own energy secretary today with an announcement in The Commons about gas and electricity deals.
Mr Cameron told MPs that suppliers will be forced to move customers onto their cheapest available contracts under new legislation.
But consumer groups have told ITV News that such a plan would limit competition and the lower prices that can bring. And questions have been raised as to how the Government could enforce such a pledge.
The Prime Minister's comments follow announcements made by British Gas, npower and Scottish Power's outlining a rise in prices.
"I can announce that we will be legislating so that energy companies have to give the lowest tariff to their customers," he told MPs during Prime Minister's Questions.
Business Editor Laura Kuenssberg reports:
Consumer campaign group Which? said the Prime Minister's pledge to ensure energy firms offer the cheapest tariffs to customers is a "big moment".
But energy group uSwitch told ITV News "it would be a mistake" to make energy firms put everyone on the lowest tariff, as that would take away choice.
Earlier Downing Street suggested David Cameron's price promise had come about after a failure by the energy companies to ensure they are offering the best deals.
The Prime Minister's spokesman said: "We have asked energy companies to take action themselves and make clear what the lowest available deals are.
"The point is, in practice this market is not operating for everyone. A small minority of people are actually switching deals, therefore we need to push some of this responsibility on to the energy companies."
The spokesman sidestepped questions about whether the announcement had come as a surprise to Energy Secretary Ed Davey.