Skipton and Portman societies said they had received resolutions demanding a demutualisation vote, while a statement on the carpetbagger.com website said the group had also collected enough signatories to force a vote at Chelsea building society.
Skipton, Britain's fifth-largest building society with almost pounds 6bn of assets, accused the carpetbaggers of seeking a short-term windfall at the expense of long-term value for customers.
Portman last night said it would no longer accept applications for new savings accounts, and promised to fight any moves to turn it into a bank. It signalled it hoped to disallow the resolution, rather than have to endure a ballot on windfalls for the society's 1 million members.
Its deputy chief executive, Robert Sharpe, said: "If the resolution proves to be valid then we will vigorously defend our mutual status."
The carpetbaggers' success came just before the rules are tightened to make it harder to demutualise. From today 500 signatures are required to force a vote, up from just 50.Reuse content