Four of the auditor’s former partners were serving on the Financial Reporting Council’s conduct committee when it decided not to investigate their former firm’s role in the bank’s collapse. Another committee member had advised KPMG previously.

The FRC, which last week emphasised the importance of transparency in its workings, has appointed Fieldfisher, a law firm, to fight a tribunal appeal aimed at winning access to documents and emails under the Freedom of Information Act.

The regulator is under pressure to improve its investigatory processes after several corporate collapses where the auditors failed to spot problems. Last week Greg Clark, the business secretary, promised an independent investigation into the regulator.

Some concerned investors say that the FRC is soft on auditors because it has been “captured” by the accounting profession, with its board and decision-making committees liberally sprinkled with former Big Four accountants.

MPs described the regulator’s initial decision not to investigate KPMG as “a serious mistake”. Poor accounting and accounting rules have been cited as one reason why no one understood how bad the bank’s problems were until it was too late. The bank was rescued by Lloyds TSB with £20 billion of backing from taxpayers. Later £53 billion of its loans went sour as the extent of its reckless approach to creditors became clear.

The FRC belatedly investigated, only to find the auditor not guilty of any serious failings — triggering more astonishment from some MPs.

Margot Gibbs, a researcher originally backed by Greenpeace, is appealing against a decision by the Information Commissioner in November in order to establish how individual members of the conduct committee voted and whether there was any lobbying between KPMG partners and their former colleagues and advisers on the committee.

She also wants to challenge the FRC claim that, despite being a public body, it is largely exempt from the Freedom of Information Act. The watchdog and the business department, its sponsoring ministry, have been fighting the public sector classification for 14 years.

The former KPMG partners on the ten-member conduct committee were Paul George, a partner until 1999, Sean Collins, one until 2009, Joanna Osborne, a partner until 2011, and John Kellas, one until 2004. In addition, Richard Fleck, its chairman, is a consultant with Herbert Smith, which used to advise KPMG.

The minutes of the meeting show that Ms Osborne and Mr Collins left when KPMG was discussed, according to a report into the affair published by the FRC in November last year. Mr Kellas stayed but “did not participate”. The report did not say what Mr George and Mr Fleck did, nor how anyone voted.

The FRC confirmed that it had appointed lawyers. “We took this approach, ie explaining why the request was out of scope and referring Ms Gibbs instead to information we had published in connection with her request, for consistency of treatment and fairness with all other FOI requesters whose requests are out of scope.”

Fines by the FRC last year were their highest ever at almost £15 million.

KPMG has been auditor to several leading British companies that have failed or come to close to failing, including Co-operative Bank, Carillion and Conviviality, the group behind Wine Rack and Bargain Booze.