The energy company employed Accenture in 2001 to design and implement the system, dubbed Juipiter, in 2006 to 2007.
British Gas said in a statement that it became apparent there were problems with the system, which cost about £300m. An independent analysis concluded that Accenture was responsible for fundamental errors, it said.
British Gas said it was forced to make additional investments and employ extra staff to help deal with the system's failures.
British Gas parent company, Centrica, has issued a high court writ claiming that the inability to properly bill customers was catastrophic.
Accenture said that the claims are baseless and without merit and it will put up a vigorous defence.
The company said in a statement that the system it delivered met all its commitments, and was delivered on time and within budget.
Accenture said Centrica conducted extensive testing of the system before formally accepting it and had no basis for a warranty claim.
"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," the statement said.