A Labour AM is facing an inquiry by the Welsh assembly's Independent Standards Advisor over allegations she misused public money.
Joyce Watson denies wrongdoing over mail sent to party members
It is alleged Mid and West Wales AM Joyce Watson used assembly stationery and the assembly postal service to mail two newsletters to 1,000 party members.
AMs' code of conduct prohibits them from using assembly resources to produce or distribute unsolicited mail.
Ms Watson said, if she had made a mistake, she would work to rectify it.
BBC Wales understands the Conservatives are set to lodge a formal complaint against Ms Watson, who was elected last May.
Ms Watson has confirmed she produced the newsletter and a second similar one and posted both of them to around 1,000 Labour members in mid and west Wales using the assembly's pre-paid envelopes.
The glossy full colour newsletters contained pictures of Ms Watson and articles about her work as an AM.
Ms Watson insisted the newsletters were not "party political" because they did not mention she is a Labour AM, and she maintained her staff had cleared the mailing with the appropriate assembly authorities.
In a statement Ms Watson said: "The mailings in question contained a notice of my surgeries in the Mid and West Wales region and a summary of my work as an Assembly Member.
"No mention of the Labour Party was made and the content was approved by the relevant authorities at the National Assembly.
"If a mistake has been made with the distribution arrangements I will of course work with the assembly's authorities to rectify the situation".
The AMs' code-of-conduct says assembly resources should not be used for unsolicited material including leaflets, pamphlets, greeting cards or similar communications not requested by the recipient.
It also says use of centrally provided stationery and postal resources should not be used for circulars - which it defines as communications sent in identical or near identical form to numbers of addresses - newsletters and annual reports.
The Assembly Commission, the corporate body for ensuring property, staff and services are provided for the assembly, declined to comment.