Ministers have been told to face the Commons over the Prime Minister's surprise announcement on energy bills.

Commons Speaker John Bercow has granted shadow energy secretary Caroline Flint an urgent question on the plans, which Labour said are "unravelling by the minute".

David Cameron announced during Prime Minister's Questions on Wednesday that energy companies would be required in law to give customers the cheapest available deal.

However, there was little detail about how such a scheme would work and the Department for Energy and Climate Change appeared to have been taken by surprise by the announcement. Energy Minister John Hayes will answer the urgent question, said a departmental spokesman.

Wednesday's announcement follows a string of above-inflation price hikes by major energy companies in recent days. Ministers have previously announced moves to require energy companies to inform their customers if they could be on cheaper tariffs. But Downing Street said the forthcoming Energy Bill would go further by introducing a requirement for companies to give people the best tariff for their circumstances.

The move is intended to tackle concerns that many householders, at a time of rising fuel costs, are already paying more than they need to because of the bewildering number of tariffs available.

Asked on Wednesday whether Mr Davey knew about the announcement before it was made, the Prime Minister's spokesman said: "Clearly there have been discussions about this issue and those discussions have been going on for some time."

Consumer campaign group Which? said that the announcement was a "big moment" for consumers and an acknowledgement that competition in the energy market had "failed".

But consumer website uSwitch warned that the initiative could backfire by reducing choice for customers.

Director of consumer policy Ann Robinson said: "The unintended consequences would be to kill competition. Consumers will be left with a Hobson's choice - there will be no spur, no choice, no innovation and no reason for consumers to engage anymore."