The civil action has been brought against the construction firm Carillion, which bought spa building firm Mowlem in 2006. A 'Part 20 notice' has also been served against architectural practice Nicholas Grimshaw and Partners (NGP), which will involve them as a party to the litigation.

A statement issued by BANES said: "The council's position is that significant costs which it incurred in relation to the Bath Spa are the result of design and/or workmanship issues for which Carillion and/or NGP are responsible."

The council is bringing the £21m counter-claim against Carillion after the construction company served a writ against the council for £2.4m in August 2008. According to BANES, the council was then told to makes its response to the High Court by 5 December, and, on that date, lodged counter-claims with the court for £21,366,318 plus costs.

The much-delayed £45m (US$68.8m, 50.3m euro) Thermae Bath Spa opened in August 2006, more than four years after its scheduled opening date and nearly three times over budget.

Plans for Thermae Bath Spa were first revealed in 2000 but construction was beset by delays including a problem centring on faulty paintwork on the spa’s pool walls and another fault in the waterproof floors of the steamroom.

BANES said it had sought to resolve the dispute "constructively" with Carillion, "without recourse to the courts".

Its statement said: "The council has consistently maintained that it has no liability to Carillion but rather that it has significant claims against them and/or other parties in connection with design and/or workmanship and/or supervision related to the Bath Spa project.

"At the same time, Nicholas Grimshaw and Partners (NGP) has also been served with a Part 20 notice which will involve them as a party to the litigation. Carillion has made allegations as to design failures in relation to the spa against NGP and the council will be passing on these allegations and/or otherwise holding NGP responsible for design and/or supervision failures in relation to the project."

In response, an NGP spokesperson said: “We have received notice that the operator of the Bath Spa project has filed a suit. It is not, however, our policy to comment on potential or actual litigation.”

A Carillion spokesperson declined to comment, saying that the company would not comment on contractual matters as they are "commercially sensitive". Details: thermaebathspa.com or bathnes.gov.uk