At a campaign event before the May 6 election, the party released its "green manifesto,” fleshing out its plans to promote competition in the energy sector and to encourage progress to a low-carbon economy.

It said it wanted to open up the retail energy market, dominated by the "big six" energy companies, to new entrants to give consumers greater choice.

If reforms proposed by Britain's energy regulator, Ofgem, did not succeed, Labour said: "We are prepared to legislate to ensure greater competition in energy supply." "We will consider measures such as limiting the ability of energy companies to generate electricity solely for themselves and reducing the barriers to the changing of licence conditions," it said.

Ofgem said in February that if new market-based initiatives to increase wholesale power market liquidity did not work, it might have to reform the market, possibly by forcing vertically-integrated utilities to sell some of their output to independent suppliers, to increase competition.

Britain's power market is dominated by six large utilities - Scottish and Southern Energy, Centrica, RWE npower, EDF Energy, E.ON UK, and Iberdrola's Scottish Power.

Labour said in the document released on Sunday that it would examine whether Ofgem itself needed further reform.

Labour said that, under the just-approved Energy Act, it would require energy companies to provide discounted tariffs to vulnerable customers.

This would mean that a million low-income pensioners aged over 75 would receive at least 100 pounds ($155) off their annual energy bill by 2013, it said.

The 100 million pound cost of the measure would be funded from a levy on energy companies, Energy Secretary Ed Miliband told Reuters.

Prime Minister Gordon Brown's Labour Party has ruled Britain for 13 years but lags the opposition Conservatives in opinion polls. Some polls suggest Labour could come third in public support behind the Liberal Democrats, usually regarded as Britain's third largest party.

However, the polls suggest a "hung parliament,” in which no party wins an overall majority in parliament, is likely.

In the new document, Labour also pledged to introduce a statutory code of consumer rights, requiring energy firms to give information on the cheapest available tariffs on bills sent to customers.

An ombudsman would have the power to force companies found guilty of breaches of the code to compensate consumers directly, it said.