The local councillor responsible for Grimshaw's controversial Bath Spa project has vowed to win back the overspend through the courts.
Grimshaw could face a legal tussle with Bath and North East Somerset Council over who must pay for the spa's spiralling costs.
An independent audit by PricewaterhouseCoopers shows costs on the scheme have soared to £26 million - well in excess of the original £19 million estimate - and further expenses could be incurred before it is finished.
Furious councillors are looking to claw back the overspend and may take legal action against Grimshaw and the project's contractor, Mowlem, if they are shown to be culpable.
'We are talking about a lot of public money coming back to us, ' said Nicola O'Flaherty, the council's executive member for tourism, leisure and culture.
'We might file a claim but not until the project is finished.
Mowlem has made a number of claims against the council. We have agreed to some but we do not recognise a large amount of them.'
Mowlem has promised that the centre will open this autumn, but O'Flaherty says she is not 100 per cent confident the contractor will meet its pledge.
Although Grimshaw has refused to comment, it has told O'Flaherty that the problems are 'within the norm for a restricted site of archaeological significance involving spring water'.
Earlier this year an investigation into peeling paint on the Bath Spa scheme appeared to criticise Grimshaw, Mowlem and product RIW Toughseal (AJ 5.2.04).