USERS of gas in Britain will pay an extra £10 billion this year because energy markets in continental Europe are failing to liberalise fully, the Liberal Democrats will tell ministers this week.

The figure — which is half the total gas supply bill for the UK — has been calculated by Global Insight, the energy consultancy, in a report for Centrica, the gas and electricity provider.

Global Insight said that the costs were arising because “lack of liberalisation has meant that the pricing of gas on an oil-indexed basis across Europe has been maintained, isolating the level of European gas prices from underlying supply and demand dynamics”.

The Liberal Democrats intend to step up calls for the Government to press Europe for further liberalisation after receiving the report. Ed Davey, the party’s trade and industry spokesman, said: “UK customers are paying a heavy price for Europe’s failure to sufficiently open its energy markets as we become increasingly dependent on gas imports.

“In addition, the Government has manifestly failed to promote energy efficiency savings and the benefit this can bring to UK businesses and environment.”

The Global Insight report said that since the opening of the European gas interconnector, British gas spot prices had been highly influenced by movements in European gas contract prices, which were directly linked to changes in oil and oil products. But even with that link, the spot market remained responsive to supply and demand patterns and there was no evidence that the spot market itself was not working efficiently.

The report said that the forward gas markets had been “highly bullish and at times unpredictable” over the past two years and that price movements had been “inconsistent with liquid and deep forward market behaviour”.

Trevor Sikorski, the lead author of the report, said: “Our analysis shows that there can be little doubt that UK consumers are paying the price for Europe’s failure to sufficiently open its energy markets as we become increasingly dependent on gas imports.”

The warning comes amid continued concern from business over soaring costs. This month Corus, the Anglo-Dutch steelmaker, said it expected its energy bill this year to be up to 35 per cent higher than last year, which itself was 35 per cent higher than 2004.

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