The assembly leader of UKIP leased an unusable office riddled with damp without a survey and against advice from solicitors, a probe has concluded.
Standards commissioner Sir Roderick Evans believes Gareth Bennett breached the AMs' code of conduct when he spent almost £10,000 in public money on a failed Pontypridd office project.
Leaked draft findings state the AM did not visit the property until months after the lease was signed.
Mr Bennett declined to comment.
BBC Wales revealed in April that the AM, who was not leader at the time, had abandoned a scheme to open a constituency office at the former Angharad's nightclub in Rhondda Cynon Taff.
The lease was due to last until April 2021 but it was terminated in September 2017 after it emerged the cost of repairing the building exceeded the allowances available to AMs.
Mr Bennett has apologised to the standards commissioner.
The draft findings, seen by BBC Wales, show the standards commissioner believed Mr Bennett had been "reckless".
Sir Roderick found that £9,883 had been spent on the property, including £5,200 in rent and £2,477 in building materials.
Mr Bennett signed the lease in March 2017 but did not visit until the following July.
That summer, staff realised that the property, which had no electricity on the first floor, had damp on two floors and was in a state of disrepair.
But no survey or an estimate of the cost of the work had been obtained before the lease was signed.
After a survey was paid for by the assembly, damp was found to be extensive throughout the property.
Sir Roderick was also given evidence from Mr Bennett's solicitors that he had been advised against the scheme.
The commissioner was unable to say with certainty what had happened to the building materials purchased, although he concluded at least some had been used in the office.
Mr Bennett had told the standards commissioner that a builder had misunderstood he was awarded work in the office and had built some partition walls.
A member of staff who gave the builder access went on leave shortly after the lease was signed and did not return to work.
Sir Roderick had no suspicion that Mr Bennett derived personal benefit from the cash paid out.
Mick Antoniw, Labour AM for Pontypridd, said he could not comment on the standards commissioner's investigation or his conclusions.
"But on the basis of what is in the public domain already, my view is he has wasted an enormous amount of public money on a non-functioning office, has done nothing to serve the people of Pontypridd and if he has any decency he will voluntarily repay all the public money out of his own pocket," he said.
The matter is set to be passed to the standards committee and AMs could choose to exclude Mr Bennett from the assembly for a period of time without pay.
A spokesman for Gareth Bennett said: "We don't comment on politically motivated malicious leaks."
The spokesman previously said Mr Bennett paid £4,500 "out of his own pocket to ensure the lease was terminated".
The investigation was triggered after the chief executive of the assembly, Manon Antoniazzi, referred the matter to Sir Roderick.
An assembly spokesman said the institution was unable to comment and Sir Roderick declined to comment.