Henk Verschuur, managing director of the spa's operator Thermae Development Company (TDC), made the revelation on the BBC's Inside Out West programme, screened yesterday evening in the West Country.
'In hindsight I wouldn't have done it', he said.
Duncan Bannatyne, a millionaire businessman and leisure expert who fronted the programme, added to the chorus of disapproval, saying that the only way the project could turn a profit would be if it was concreted over.
Bannatyne, who also interviewed the Spa's architect Nicholas Grimshaw (pictured) during the broadcast, said: 'I've tried to be optimistic, I've tried to think about how best to do it, but really the best thing you could do is knock it down and build a multi-storey car park.
'It's the only way you could make it pay. Someone's got to subsidise it.'
The entrepreneur went on to predict that the leisure project, which is due to open in April 2006, will survive for a maximum of five years depending on how long TDC 'throws money at it'.
He added: 'If you gave it to me for absolutely nothing, I don't think I could make it pay.'