Demolition of a leisure centre will cost an extra £1m and has been delayed by 18 months after asbestos was found.
A survey of Abertillery Leisure Centre missed the brown asbestos. A report said cavity walls had been checked.
Blaenau Gwent council said it was considering legal action against the firm behind the survey.
A spokesman for the company said it had cooperated fully with the council to try and resolve the issue since it was first brought to its attention.
The matter is now in the hands of its insurers.
Blaenau Gwent council has suspended demolition work until a new firm has removed the asbestos, at a cost of £998,992.
Despite a specialist survey being carried out to check for asbestos, it was not mentioned in the report and was only discovered once demolition work had begun, the council said.
The council said a consultant was hired to carry out a pre-demolition refurbishment (PDR) survey to identify the presence of asbestos.
A spokeswoman said “other sources of asbestos not identified within the PDR were discovered within building cavities and other areas”.
She added: “This required a suspension of works to allow a new survey to be undertaken. Based on its findings a far larger, more complex programme of work was identified and developed before the work could be tendered.
“The authority is currently seeking legal advice regarding potential action and cost recovery.”
Demolition work was set to cost just under £400,000, but the council has now awarded a contract costing £998,992 to remove the asbestos before the building can be torn down.
Asbestos removal work began in December and is set to finish in May, but the demolition is now not due to finish until August, having previously been earmarked for a March 2017 finish.
The amosite material found, also known as brown asbestos, was often used in cement sheets and pipe insulation, but can also be found in ceiling tiles and thermal insulation products.
It consists of brittle fibres which are very harmful to people if inhaled, causing asbestosis and lung cancers. Its use was banned in 1985.