Like other reluctant joiners, they expressed reservations about the new body. David Gilchrist, general manager of Halifax, said the decision had been forced by the Securities and Investments Board's wariness over regulating firms directly. There had in effect been no choice, he said.
Halifax still objects to the regulatory system. 'Ultimately we want a system administered by a single statutory body,' Mr Gilchrist said.
He said the PIA, whose board consists of public interest members and representatives of big investment firms and independent advisers, would find it difficult to manage conflicts of interest.
The society was troubled as the PIA had not yet published its rules and practices. Mike Blackburn, chief executive, said: 'The PIA stands a better chance of reaching the necessary standards if it has the commitment of large and responsible financial institutions like the Halifax.'
A spokesman for Allied Dunbar said: 'Our application is not grudging, because the PIA will work better if the big offices join. Having said that, we do not see it being able to deliver the step change in standards needed if consumer confidence in the industry is to be restored.'
Only Prudential has opted for direct regulation by the SIB. Last week Lloyds Bank and Standard Life, also sceptics, announced their decisions to join the PIA.
Mark Boleat, director general of the Association of British Insurers, said: 'For the time being the PIA is accepted, but there is a question about how the system will look in five years' time.'
The pressure applied to firms to join the PIA has led many to question the regulators' lack of accountability to either the firms that fund it or to Parliament. Jim Stretton, UK chief executive of Standard Life, said it would continue to urge the development of a more accountable structure.
Allied Dunbar also recently picked out the prospect of a 'huge increase' in enforcement costs, which the industry fears will give a competitive advantage to companies based in other EU countries.
Anthony Nelson, Treasury minister of state with responsibility for financial regulation, has indicated that the Government will take a tough line on the PIA's costs.