An IT system that was supposed to make British Gas the darling of consumers nationwide has instead become the focus of a multimillion-pound legal battle.

British Gas had hoped to consign complaints about the business to history, but in the event it was described by watchdogs as being in meltdown and thousands of its customers decided that they had suffered enough and switched to a rival.

Now the origins of the customer service problems a year ago, which caused complaints about Britain's biggest residential energy supplier to rise nearly threefold to record levels, are at the centre of a £182 million High Court writ.

Centrica, the parent company of British Gas, confirmed yesterday that it was suing Accenture, the global consultancy group, about the state-of-the-art IT system.

It claims that the “Project Jupiter” system reduced British Gas's customer billing process to such a mess that the energy supplier had to hire 2,500 extra staff and invest millions more pounds to fix the problems and make it work.

The showdown promises to last for months as each company fights to prove that it was not to blame for inaccurate bills sent to homes across the UK. Complaints to Energywatch, the watchdog, about British Gas hit a record 14,001 in March last year.

Accenture vowed yesterday to fight its corner, stating: “We are confident, based on the facts of the situation, that this claim is baseless and without merit. Centrica is only trying to shift the blame for a situation it created.”

Centrica hired Accenture to provide the new billing system seven years ago.

It was to bring together the records of British Gas's 12.5million gas and electricity customers on to one platform capable of handling 250,000 meter readings and 200,000 bills a day.

The £317million fee would come from the £397million of savings that British Gas expected to obtain from the project.

Centrica claims that, after a number of glitches, in March 2006 Accenture guaranteed a software upgrade that would work. Centrica argues that, instead, the system continued to struggle and generated a high level of “exceptions” - billing issues that required manual intervention.

Centrica also claims that Accenture failed to provide adequate computer hardware and did not integrate the system properly. The energy supplier formally notified Accenture that it was in breach of contract in February 2007.

A British Gas spokesman said: “An independent analysis of the billing system has concluded that Accenture was responsible for fundamental errors in the design and implementation of the system. British Gas has been left with no option but to pursue legal redress against Accenture.”

In the past year, since British Gas fixed the system itself, complaints to Energywatch about the supplier have fallen 85 per cent, the spokesman said.

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