Hundreds of thousands of Npower customers will find themselves moved to a new energy supplier next year after it agreed to sell their accounts to Utility Warehouse.

Both companies says nothing will change for affected customers. But if you're one of them, it's a good time to check your current deal and consider if you could save by switching and fixing (see our Cheap Gas & Elec guide for help).

Around 418,000 electricity and 357,000 gas accounts, which are part of Npower's Electricity Plus and Gas Plus subsidiaries, will be sold for £218 million to Telecom Plus, a supplier of energy and telecoms services which trades as Utility Warehouse.

Telecom Plus shareholders and the Office of Fair Trading need to approve the deal first, but it should be completed by early January.

Npower's sale comes after energy regulator watchdog Ofgem told suppliers to limit the number of tariffs on offer to domestic customers to a maximum of four.

I'm moving to Utility Warehouse. What does this mean for me?

Npower and Utility Warehouse have both confirmed nothing will change for affected customers when the deal goes ahead. So your tariff, contract, bills and customer service will not change as a result of the sale.

Utility Warehouse has actually managed Npower's Electricity Plus and Gas Plus accounts since 2006, although Npower will continue to provide gas and electricity to Utility Warehouse for the next 20 years under the deal.

If you're on a variable tariff, you'll be subject to any price rises or falls Utility Warehouse makes in future.

A spokesperson for Utility Warehouse says customers should see "competitive pricing" in the long run as a result of the deal.

Energy firms 'must explain costs'

News of the sell-off comes a day after energy and climate change minister Baroness Verma said firms will be forced to give customers a breakdown of costs, as part of amendments being made to the Government's Energy Bill.

Information on bills will include the cost of complying with Government environmental programmes, wholesale energy costs, network and distribution costs and energy company operating costs and profits.

Four of the biggest suppliers already provide this breakdown, but the Government wants other energy firms to agree to do the same. If they don't, then they will be compelled to by law, she said.

The Bill cleared the House of Lords last night and will now return to the Commons for MPs to debate the amendments.

Additional reporting by Tim Heap.