Prime Minister David Cameron faced criticism from within his party amid the continuing controversy over his watered- down energy pledge.

Conservative MP Tim Yeo, who chairs the Energy and Climate Change Committee, has warned legislation to help consumers could be delayed amid the confusion.

Energy firms came under attack from the industry watchdog, which unveiled plans to ensure customers are offered the cheapest deals. The announcement by Ofgem appears to fall far short of what the Prime Minister announced earlier this week.

Then he said the Coalition would legislate to ensure customers were put on the lowest tariff. But the promise appeared to unravel within hours.

Mr Yeo has now written to Energy Secretary Ed Davey to express concern that the move could delay the Energy Bill.

The warning came as Ofgem called for simpler, clearer, and fairer energy. Its reforms would force energy firms to tell customers about the best electricity and gas deals and cut the number of core tariffs to four per fuel type.

But critics have warned just telling customers about tariffs is far short of ensuring they are charged the cheaper rate.

Consumer groups broadly welcomed the regulator's proposals, but Labour accused Ofgem of "tinkering at the margins".

Ofgem chief executive Alistair Buchanan said: "Our plans will put an end to consumers being confused by complex tariffs and will usher in a simpler, clearer, fairer and more competitive energy market for all consumers."

The regulator is consulting on its proposals, but could introduce its reforms by the summer of 2013. It proposed suppliers should offer four core tariffs per fuel type to cut the baffling array of deals on the market.

The Prime Minister plunged energy policy into confusion with his surprise announcement that the Government would legislate so that gas and electricity companies have to give the lowest tariff to their customers.

Mr Davey distanced himself from the proposal and No 10 said energy firms would be obliged only to offer the cheapest tariffs.

Mr Cameron, attempting to clarify his statement, stopped short of repeating his previous suggestion that companies would be compelled in any way to put customers on the cheapest tariffs.