British Gas is suing Accenture for £182m in costs connected to the failure of a new billing system put in place by the consultants in 2006.
Problems with the system led to a massive increase in complaints against the gas supplier. Centrica, British Gas's parent company, has already written off £200m due to problems with the system.
Complaints about British Gas peaked in April 2007. By October 2007 energywatch figures showed it was still the second worst energy supplier for customer service.
British Gas said complaints have fallen 85 per cent since then.
It said: "British Gas sought to establish what went wrong and why. A subsequent independent analysis of the billing system has concluded that Accenture was responsible for fundamental errors in the design and implementation of the system.
As a direct consequence British Gas was forced to make significant investments to address the system failures and these investments are ongoing. It also incurred significant additional staff costs to manage the customer service issues. The increased cost to address the problems comes on top of the initial £300m investment." The company said it had no other option but to sue Accenture.
It claims it had to hire 2,500 extra staff to help sort out problems created by the billing system.
Accenture sent us the following statement: "Accenture rejects responsibility for the situation Centrica created. Centrica directed the design, build and implementation of the Jupiter system and insisted on many of the features they now find problematic. At their own choice, after extensive testing, in March 2006 Centrica took over total control over all aspects of the system about which they now complain and has operated the system themselves for over two years...We are confident based on the facts of the situation that this claim is baseless and without merit. Accenture will vigorously defend the High Court proceedings."
The statement said that Centrica was trying to shift blame for a situation it created.®