The fire safety certificate for cladding on a block of Cardiff flats has been found to be fake.
A resident of Marseilles House at Century Wharf discovered the building's External Wall Fire Review (EWS1) contained a false surveyor's signature.
An industry-wide certificate was introduced following the Grenfell Tower fire in 2017, in which 72 people died.
The Welsh Government said it was "very concerned" at the allegations, which are being investigated.
It was only thanks to a suspicious former policeman that the forgery was discovered.
Flat owner Gareth Griffiths, 66, became suspicious over the EWS1 that was signed in July for the block of flats in the Butetown area.
"When I looked at it, the writing didn't look great for a professional RICS [Royal Institute of Chartered Surveyors] surveyor," he told the Local Democracy Reporting Service.
"When I rang the surveyor, she was horrified that the survey took place with her signature."
The surveyor, who did not wish to named, confirmed she had not carried out the survey or signed the form.
Property management company Warwick Estates contracted Newbridge-based Specialist Facade Inspections to conduct the EWS1 survey.
However Specialist Facade Inspections said it only produces a report - which is signed off by a third-party chartered surveyor for a fee of £200 - and it had also been the victim of fraud.
The company's founder Paul Tedstone said the forged signature was also used on "five or six" other EWS1 certificates, but could not provide contact details for the third-party surveyor or the name of the company.
"EWS1 is a bloody piece of paper. In order to tick that box you need a [qualified person]. None of those accreditations I hold, nor did the business hold," he said.
"It was nonsense: you need someone else just to confirm what we already know. Just for the last piece of paper, as I didn't have the letters after my name to sign it off.
"There has been weeks and weeks of intrusive surveys. We have done nothing wrong, apart from being a bit naïve. We're victims too."
Mr Tedstone said the company has withdrawn every certificate with the false signature on, reviewed each case and reissued each certificate.
Warwick Estates has also been asked to comment.
The scandal prompted Neil McEvoy MS to raise concerns at the Welsh Parliament of a wider problem.
"The person who supposedly signed off this safety certificate has stated in writing that they did not carry out the inspection, and they did not sign the form," said Mr McEvoy.
"They have no connection to Specialist Facade Inspections and the signature on the letter is not [hers].
"Specialist Facade Inspections say they're the victim, but the bottom line is we have a safety certificate which I don't know who it has been signed by. This really is a pressing issue.
"When will the housing minister get a grip on matters? Set up a task force and sort this out."
The Welsh Government said it was aware of the allegations, which were being investigated by the "appropriate authorities".
A spokesperson added: "We are very concerned about any alleged fraud in relation to such a vitally important issue."
EWS1 forms must only be completed by competent chartered professionals with suitable fire expertise.
However the current high demand for these certificates in the wake of the Grenfell disaster has led to scammers issuing fake EWS1 forms, according to consumers association Which?.
"Scammers have forged the names and signatures of qualified surveyors to pass and fail buildings. Some forms we've seen have been signed off by surveyors who simply don't exist," the report said.
The RICS said it condemned such fraud which could have "potentially dangerous consequences" for residents.
Meanwhile, the residents of Century Wharf remain unsure whether the cladding on their building is safe.
Mr Griffiths said: "Until it's properly surveyed, we don't know whether combustible material is in the walls or cladding outside the building. I'm very concerned about the safety of the building."