The National Federation of Retail Newsagents has written to prime minister David Cameron to officially complain about what it claims is the "inappropriate conduct" of public health minister Anne Milton.

In a letter seen by The Grocer, new NFRN national president Kieran McDonnell wrote that at an All-Party Parliamentary Group on Smoking & Health's 40th anniversary event for ASH last week, Milton had credited the vice chair of the group, Kevin Barron MP, for his help "behind the scenes" when addressing smoking legislation, and also accepted and presented awards. Her comments, McDonnell wrote, "called into question the manner in which recent tobacco display ban legislation has been made; and the minister's ability to be considered unbiased".

"I would ask that you formally conduct an investigation into the conduct of your minister in the light of her public admission she had worked with an officer of an ASH-funded parliamentary lobby group on recent legislation "behind the scenes", he said in the letter. "Moreover, this inappropriate conduct necessitates a review of the legitimacy of the legislation."

The complaint comes four months after the NFRN was itself the subject of an investigation by The Observer that claimed its campaign against the display ban was being directed by tobacco companies.

An NFRN spokeswoman insisted it was not concerned its complaint could be seen as retaliation.