Phyllis Bowman, the political director and founder of the Society for the Protection of the Unborn Children, which has over 50,000 members, has resigned from her post.
Nearly half of the 12-person national executive of the organisation has resigned in sympathy.
It is believed that Mrs Bowman quit because of a rift with John Smeaton, the national director, over the future strategies of the organisation.
Mrs Bowman founded the SPUC 30 years ago and has twice re-mortgaged her home to keep it going. In the past, the organisation has sent humanfoetuses to MPs and held marches against abortion.
The Westminster All-Parliamentary Pro-Life group, which includes the Shadow home secretary, Ann Widdecombe, has written a letter to SPUC saying that it is "distressed and alarmed" at recent events which led to the resignations.
The group says in the letter: "We question the probity of this situation and will be sharing our concerns with others in a position to take appropriate action to ensure that donations to the political fight are used for that purpose."
The society is mainly funded by individuals but the parliamentary group fears that resources will be diverted from the political arena towards developing a closer relationship with the Roman Catholic church.
They believe that this could alienate the large number of members who are Protestant, Muslim and atheist, and so lose support for its campaigns against abortion, euthanasia, and embryo experiments.
"We are appalled that a decision of this magnitude - the diversion of the whole society from its original strategic objectives - has been undertaken without proper consultation with its membership, the largest part of which, we know from our own contacts, is unaware of this change of direction," the Westminster All-Parliamentary Pro-Life group said in its letter.
Mr Smeaton said that he was surprised by the parliamentary group's reaction and that all organisations had to "move on". He said: "We had a vote of the 65-person council and decided on the way forward for the new millennium.
"Mrs Bowman is staying on as a trustee of the organisation and the parliamentary focus will remain central to our work, as it has been for the past three decades. We are meeting the parliamentary group next week to sort this out."Reuse content