The Association of British Insurers (ABI) has written to senior non-executive directors at more than 20 leading companies asking them to explain why the company has one person filling the roles of both chairman and chief executive.

A spokesman for the ABI said: "Separate roles are regarded as best corporate governance practice. We asked the companies to explain to us why, in their company, the two roles were combined."

Concerns about boardroom power were highlighted by the Tomkins case where Greg Hutchings, who had previously held both roles, resigned as chief executive after stories about the inappropriate use of company jets surfaced.

The ABI spokesman said the letters had been planned before the Tomkins case came to light but the case had given the move some added impetus.

The ABI would not confirm the names of the companies involved, but they are thought to include the Alliance & Leicester, Marks & Spencer, Powergen and insurance company Old Mutual.

The spokesman said that no more letters were planned and those companies contacted were "the ones we thought it was appropriate to write to". He said the ABI had received few replies and would be talking to the companies concerned, but the ABI would not be commenting on individual companies.

ABI members own around a quarter of all UK shares. The ABI said it would continue to try and use its influence behind the scenes. "We're back to our 'quietly, quietly' role," the spokesman said.

The issue of corporate governance came to prominence with a report from the Cadbury committee eight years ago. It recommended the separation of the offices of chief executive and chairman. Two years ago a code on corporate governance was produced, combining Cadbury's recommendations with those from other reports, but the code has no legislative force.