Vale of Glamorgan MP and Welsh Secretary Alun Cairns has been criticised for mailing out a sticker to constituents front door warning political canvassers from cold calling.
The sticker lists campaigners alongside uninvited traders, charity cold callers and religious groups.
Vale of Glamorgan Labour is to complain about the letter, accusing Alun Cairns of trying to block rivals from speaking to voters.
A spokesman for Mr Cairns told BBC Wales he wanted to provide his constituents with a "proven means of reducing the number of doorstep callers".
Alun Cairns was first elected as MP for the Vale of Glamorgan in May 2010. In the 2017 general election he was re-elected with 2,190 votes between him and the Labour candidate.
The sticker is accompanied by a letter on House of Commons notepaper, and was sent out in an official Commons envelope.
"These preventative notices have dramatically reduced the number of cold callers for those already using them," Mr Cairns wrote in the letter.
Labour Vale of Glamorgan prospective candidate Belinda Loveluck-Edwards said: "It appears that Alun Cairns is using public money to try and block his political opponents from speaking to voters. This must be investigated.
"This is a deliberate attempt to present a false equivalence between genuine political canvassing and those who would wish to scam vulnerable people."
Labour is making a complaint to the parliamentary standards commissioner about the letter.
Under the code of conduct MPs cannot use public resources in a way that would "confer undue advantage on a political organisation".
A spokesman for Mr Cairns said: "There have been a number of complaints about the increase in nuisance callers and the rise of the doorstep economy in the Vale of Glamorgan.
"Ahead of the clocks going back, Alun wants to provide constituents with a proven means of reducing the number of doorstep callers."