Mumsnet has called on ministers to review HM Revenue and Customs' contract with Bounty, which hands out baby bags to new mothers on maternity wards containing free samples of products and a copy of the child benefit form.
The firm is paid £90,000 a year to give out the form.
In a letter to Treasury minister David Gauke, Mumsnet said the Government should cancel its contract with Bounty.
A survey by Mumsnet around the same time found 17% of mothers felt that Bounty reps had implied they would not be able to claim child benefit if they did not hand over their personal data, a spokeswoman for the website said.
Mumsnet chief executive Justine Roberts said: "There are several more practical and much cheaper ways to distribute child benefit forms, which are freely available online, including having them given out when children's births are registered - a method which would reach 100% of parents."
Clare Goodrham, general manager of Bounty said: "Once again, we are the target of a malicious campaign by a direct commercial competitor.
"Unlike Mumsnet, Bounty doesn't view the essential distribution of child benefit forms as a 'dubious' service, which is why we are proud to help all mums across the country access child benefit.
"According to the HMRC, 82% of all UK child benefit is paid as a result of the forms handed out by Bounty."