Michael Martin chairs the committee which oversees MPs' expenses

Spokesman Mike Granatt says he was misled by Commons officials over a Mail on Sunday story about the Speaker's wife using taxis for shopping.

He said he quit for "ethical reasons" after "unwittingly" giving out the wrong information to a journalist.

It comes as Parliament's standards watchdog probes a complaint over Mr Martin's alleged use of air miles.

Mr Granatt, who has worked as a top-level communications specialist for more than two decades, was adamant that Mr Martin was not at fault over the misinformation.

Instead he blamed officials, claiming they had not told him the entire truth.

"I have stepped down from my post for ethical reasons, because I misled a journalist unwittingly," said.

"I learned on Friday that I had been led to mislead journalists over material facts in a story concerning the Speaker's household and the use of taxis."

'Embarrassing'

At the end of last year, it was reported that Mr Martin's wife had claimed more than �4,000 in taxpayer-funded taxi expenses since May 2004.

If it's a case that he was simply misinformed by someone else then that hardly seems to be his fault and he seems to have resigned prematurely Lib Dem MP Norman Baker

At the time, Mr Granatt rebutted the story, saying the shopping trips had been for food for government-related functions, and Mrs Martin had been accompanied by an official at all times.

But Mr Granatt learned from the Mail on Sunday that the official was actually a housekeeper from the Speaker's household.

"It's embarrassing and he's decided because he effectively misled a journalist over this, he's had to resign," said BBC political correspondent Ben Wright.

Mr Granatt's resignation is the latest controversy to hit Mr Martin.

He has been accused of flying members of his family business-class from Glasgow to London for a New Year break, using air miles gained from official trips.

Inquiry

Mr Martin chairs the Commons Members Estimates Committee, which oversees MPs' expenses.

He is heading a "root and branch" parliamentary inquiry into all aspects of MPs' expenses, in the wake of a furore over Tory MP Derek Conway's employment of his sons.

Liberal Democrat MP Norman Baker said he was "surprised" by Mr Granatt's resignation.

"If it's a case as he suggests that he was simply misinformed by someone else then that hardly seems to be his fault and he seems to have resigned, in a way, prematurely," he said.

"Obviously, he's a dedicated official who takes it seriously that he has given out information that was incorrect."

Mr Granatt is widely regarded as an expert in communications.

He joined the civil service in 1979 and held several important positions, including head of the Government Information and Communication Service (GICS) and its crisis management unit.

He also served as the government's counter-terrorism advisor but in 2004, he joined public relations consultancy Luther Pendragon.