Serge Lourie, the founder of the Halifax Action Group, fears that a decision to recommend the go-ahead will be rushed through before Christmas.
He has written to the OFT to ask it to recommend that the proposal is referred to the Monopolies and Mergers Commission.
He has the backing of Austin Mitchell, MP for Great Grimsby, who has also written to the OFT asking it to look at the wider public interest rather than just competition.
He fears that the OFT plans to rush out a recommendation on the merger before Christmas. "To want it all over by Christmas makes merger the kind of bum's rush appropriate to dodgy organisations with something to hide," he said.
He considers it vital that the OFT takes the widest possible view of the plans, which are only the first stage on the way to demutualising and operating as a bank with a stock market listing - like Abbey National.
The Action Group has found over 200 people who are interested in opposing the merger. There are no set membership fees but Mr Lourie plans to ask for contributions to a fighting fund and suggest something like £5 to £10 per person.
He is a veteran of the protest against the withdrawal of European Ferries' shareholder perks. He is a Liberal Democrat councillor and has been involved in housing associations.
Michael Heseltine has to make a decision on whether to refer the merger to the MMC, and the convention is that he decides within six working days.
Halifax Action Group, 59, Burlington Avenue, Richmond, TW9 4DG. Tel: 071-828 0361 or 081-876 3545.