Fraudsters posing as government tax officials are using Welsh Assembly phone numbers to threaten people, who are then making panicked return calls.
An automated phone scam attempts to con people out of their money by threatening them with legal action.
One assembly employee said they had been inundated with calls from people ringing the numbers.
The assembly has informed the National Crime Agency and Action Fraud of the issue, a spokeswoman told BBC Wales.
The automated message claims HM Revenue and Customs (HMRC) is filing a lawsuit against the recipient, and urges the caller to press a button to speak to a caseworker to make a payment.
“Some are phoning back quite upset and confused,” an assembly staff member said.
The problem is not Wales-specific and victims from other parts of the UK and who may never have heard of the Welsh Assembly are ringing.
“That doesn’t help,” the employee, who had “lost count” of the number of calls they had received, explained. “You are trying to tell them it’s a scam and they’ve never heard of you.”
It was a problem across the assembly estate, the staffer said.
The problem is known as spoofing – where a fraudster fools the telephone network so the recipient of a call is given a false caller ID.
The HMRC has been successful in getting the telecoms industry to bar anyone from spoofing the tax office helpline number.
But criminals appear to be using the assembly’s phone numbers, and those of other establishments and companies, which are similar to the HMRC’s own phone lines.
A National Assembly for Wales spokeswoman said: “Unfortunately, this is an ongoing issue and is part of a targeted automated phone call scam.
“We are working with HMRC and our telephone providers in order to minimise the impact and have reported the issue to the National Crime Agency and Action Fraud.
“We are also aware of similar scams that are using text messaging and we would advise anyone who is worried about the content of the messages to contact the official HMRC helplines.”