The Alliance & Leicester has decided to treat all its 2.4 million savers and borrowers the same when it comes to the payout of shares.
They will each receive 250 shares worth at least pounds 1,000 - a move that has upset some customers who have large sums invested or who have been with the society for many years.
Patrick Mountain, 63, of Somerton, Somerset, a retired businessman and a former agent of the society, said he had received dozens of phone calls.
He is urging people who are angry to write protest letters warning they will withdraw all their savings, except for the amount needed to qualify for the shares.
"I would think there are probably several hundred thousand people who are feeling bitterly aggrieved," said Mr Mountain.
"They are being treated just the same as carpetbaggers who opened accounts just before the decision to convert was announced."
In a statement, the Alliance & Leicester said it had opted for the share distribution scheme "only after careful consideration of all the possible options".
It said: "We believe our chosen scheme is the fairest and most appropriate for the vast majority of our members and reflects the traditions of the society's mutual past."
A spokeswoman said the society was sorry that what it believed was a minority of members were disappointed.
More than 70 per cent of its investors have less than pounds 2,000 in their accounts and the society believes they will be "very happy" with the choice of scheme.
Asked if possible action by customers posed any threat, she added: "We don't believe so at the moment."
Meanwhile the two co-founders of the Halifax Action Group, Serge Lourie and Peter Judge, said they would again stand for election to the Halifax board and claimed the conversion process was "taking far too long". Halifax, which has 9 million customers, is set to become a bank in June 1997.
Details of the flotation were announced two years ago and it is anticipated that borrowers and savers will receive an average of pounds 1,000 each.Reuse content