Tony Blair's two right hand men could be questioned in a new inquiry into government special advisers and linked fears that civil servants' impartiality is being compromised.

Downing Street communications director Alastair Campbell and the prime minister's chief of staff, Jonathan Powell, have both been invited to go before the Committee for Standards in Public Life.

It is one of the brand marks of this country that we have high standards

Sir Nigel Wicks Standards Committee chairman

The number of special advisers, used to develop policy and media strategy, has more than doubled under the Labour government and their salaries now cost �4.4m.

Sir Nigel Wicks, chairman of the standards committee, said the inquiry, which opened on Thursday, would look to see if the key players in government were working together for the "public good".

The controversy over Jo Moore, the former transport special adviser who suggested 11 September was a good day to "bury" bad news has pushed such questions into the limelight.

Key yardsticks

The committee will not be examining any particular incidents but Sir Nigel said recent events did mean the inquiry was "timely".

The former Treasury civil servant told BBC Radio 4's Today programme that three yardsticks would be used in the investigation:

• Whether people were properly accountable
• Whether people were giving good leadership and setting a good example
• Whether civil servants were operating in an impartial way

The government has promised to introduce a Civil Service Act to protect the independence of civil servants.

Wicks: Recent concerns make inquiry timely

That will be something considered by the committee, who want to have their say on what checks and powers such legislation should include.

Sir Nigel stressed that despite recent worries the UK had a good record of high standards in public life.

"It is one of the brand marks of this country that we have high standards," he said. "The job of my committee is to make sure those high standards are maintained."

The first witnesses on Thursday included John Major's former press secretary, Jonathan Haslam, and BBC political editor Andrew Marr.

Guest list

The committee has no power to compel people to come before it and it will up to Downing Street to decide whether Mr Campbell and Mr Powell should be questioned.

A range of former and present special advisers, academics and political commentators have been invited to give their views.

That invitation list could include Jo Moore and former transport media chief Martin Sixsmith - whose dispute contributed to the resignation of Transport Secretary Stephen Byers.

Whether they will attend is unclear - there are even newspaper reports that Mr Sixsmith is banned from giving evidence because of his promise to stay silent about the row as part of a pay-off deal.