Two years after the troubled £45m Bath Spa finally opened for business, it has now emerged that council chiefs plan to sue the construction company that erected the landmark for more than £21 million.

Long before the much-vaunted complex even greeted its first customer, Bath and North East Somerset Council (B&NES) was locked in a row with builders and architects over who was responsible for a raft of defects that had been plaguing the building.

These ranged from leaking pools to peeling paint, and since the attraction opened a catalogue of extra problems with materials have emerged – from the broken footbaths to steam-room doors that warped with the heat.

The builders at the centre of the saga, Carillion, which bought the spa building firm Mowlem in 2006, counter-blames council chiefs and the architects Nicholas Grimshaw for the fiasco and filed a claim for £2.4 million against both in August.

B&NES was told to make its response to the High Court by December 5 and on that date lodged counter-claims with the court for damages worth £21,366,318 plus costs.

The council has already spent a seven-figure sum on barristers' fees for the case and last year voted to set up a fighting fund dubbed an "exceptional risk reserve" to set aside tax-payers' money for the legal battle.

The exact amount in that fund is not currently known.

In the writs filed by B&NES, the council claims responsibility for the delays at the Spa are due to design and workmanship failings involving Mowlem and/or Grimshaw.

In 2005, Mowlem bosses were furious in 2005 after the council kicked the company off the project and brought in another contractor to finish the job.

The council said it has sought to resolve the dispute with Carillion outside court, but that it had been left with no option but to issue its counterclaims.

In an official statement, the council said: "In August 2008, Carillion served a writ against the council in respect of claims of £2.4 million relating to the Bath Spa.

"Carillion had originally indicated that its claims against the council were worth around £10 million. As a result of extensive clarifying discussions and negotiations with Carillion prior to the issue of the writ, the amount of the Carillion claim has been significantly reduced.

"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.

"Prior to the issue of the writ by Carillion, the council had sought to resolve matters constructively without recourse to the courts."

The statement confirmed the counterclaim against Carillion and said that at the same time, Nicholas Grimshaw & Partners (NGP) has also been served with a notice which involves them in the litigation.

"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 spa opened in August 2006, more than five years late and many times over budget but it has proved a massive draw to visitors.

A recent survey concluded that visitors to the spa last year contributed more than £6.5 million to Bath's economy.