If the House is truly serious about having the scrutiny done by an independent office, which is what I believed it to be, then the House does need to protect that

Elizabeth Filkin

In a brief Commons statement, Mr Martin told MPs it was a matter of "great regret" to him that the letter had reached the media and said he was looking into how it happened.

The speaker said he was now writing to ask Ms Filkin for more details about of the "pressure" put on her as she investigated complaints against MPs.

Tory pressure

The comments came after Prime Minister Tony Blair refused Conservative calls for an inquiry into the row over Ms Filkin's decision not to reapply for her post.

Tory leader Iain Duncan Smith pressed for the investigation, especially into claims that named civil servants had been involved in a "whispering campaign" against Ms Filkin.

Mr Blair said the issue should be dealt with by the House of Commons "in the proper way".

He continued: "I think it's best left to the House authorities."

'No names'

While allegations against civil servants were the government's responsibility to investigate, no specific claims had been made, said the prime minister.

"It's difficult frankly to investigate allegations against unnamed people," he continued.

Mr Blair was pressed further by one of his own backbenchers.

Senior Labour MP Tony Wright asked whether the prime minister could think of any other public institution which could choose and dismiss its own regulator.

Bell says he was head-hunted

Mr Blair said if the Commons wanted to change the rules, it had the power to do so.

Earlier, in an interview with BBC Radio 4's Today programme, Ms Filkin suggested "powerful individuals" within Parliament tried to frustrate her by encouraging a "whispering campaign" against her.

Sources have told her that senior civil servants - and even ministers - had put pressure on witnesses and attempted to discredit her inquiries.

Ms Filkin refused to name the people behind this alleged campaign but stressed opposition to her investigations was not just restricted to the Labour Party.

The whole future of Parliamentary scrutiny was now at risk, she argued.

It has emerged too that Nigel Wicks, chairman of the independent committee on standards in public life, wrote to the Commons speaker a week ago expressing concerns about the shortening of the commissioner's working week.

'Job devalued'

His letter would appear to support Ms Filkin's central allegation - that the role of the standards watchdog was being reduced in importance.

Former MP Martin Bell says he was head-hunted for the post but turned it down.

"I don't see who is going to take this job," he said.

If she thinks there is a conspiracy against her, she must be off her head

Commons commission source

But it is understood about 40 applications have been received, including one from former Guardian editor Peter Preston, although he wants Ms Filkin to continue.

'Naivety' claim

Speaker Mr Martin chairs the House of Commons Commission, which includes the Tories and Lib Dems and is in charge of appointing the commissioner.

The row has prompted the commission to hold an emergency meeting and while there has been no public comment, its members are angry.

One commission source told BBC Radio 4's World At One programme: "If she thinks there is a conspiracy against her, she must be off her head."

There was stinging criticism too of Ms Filkin for being politically naive and for having an inflated view of her own importance.

The role of standards commissioner was created in the aftermath of the 'sleaze' scandals of the early 1990s.

It involves checking complaints about the financial declarations and interests of MPs.

Peter Mandelson, Keith Vaz, Geoffrey Robinson, John Major and William Hague were among the high-profile figures to be investigated by Ms Filkin.