Last October, the UK Government set up an independent panel to look at evidence that the drug Primodos, a hormonal pregnancy test popular in the 1960s and 70s, may have caused birth defects and miscarriages.
The Association for Children Damaged by Hormone Pregnancy Tests had been told that it would be meeting with the chair of the panel next week to discuss its work. But chair Marie Lyon has now been told that meeting will not take place.
"That gives me cause for great concern. You just feel in the depths of despair, being promised that this would be a thorough investigation, that there would be full involvement. I'm just so disappointed."
The Medical and Healthcare products Regulatory Agency, which is over-seeing inquiry, says it has been advised that the planned meeting might compromise the review's impartiality.
The MHRA says that Marie Lyon will be invited to all meetings of the expert group while the association is welcome to submit any evidence.
Aberconwy MP Guto Bebb has raised the issue of Primodos families in Parliament after being contacted by a constituent who is a member of the association. He understands the campaigners concern, but does not believe the inquiry falls short of what was promised.
"The situation is one of typical frustration with the way in which government work. But I would not jump to any conclusions as to the down-grading of what's been promised by ministers."
Cardiff-based solicitor Dr Sarah-Jane Richards has acted for clients over Primodos. She believes that, if properly conducted, the inquiry can get the answers campaigners want.
"It's then for the inquiry and the campaigners to decide how they are going to use that information and whether it might give them the evidence and support they need to bring their legal case."
Legal action against Schering - the original manufacturer of Primodos - was halted in 1982 for lack of evidence.
Schering was subsequently taken over by Bayer, which says that since then no new scientific knowledge has emerged to challenge its position that there is no link between the use of Primodos and birth defects.