The prime minister has accepted calls for the appointment of a government outsider to draw up new rules for the conduct of special advisers.

Number 10 confirmed it would back the move on Thursday as it set out its response to a report by the Wicks committee on standards in public life.

However, Tony Blair has decided not to accept a recommendation to allow parliamentary oversight of the executive powers that can be exercised by up to three civil servants in Number 10.

Following the departure of Communications director Alastair Campbell, these powers to give orders to civil servants are currently exercised solely by chief of staff Jonathan Powell.

Downing Street confirmed that the new adviser would be responsible for setting out rules that ministerial special advisers should comply with.

The ethics adviser will provide ministers and permanent secretaries with an additional source of professional advice as required on the handling of complex financial issues, said Number 10.

But calls for a limit on the number of special advisers that can be appointed were rejected.

Following the spat between senior civil servants and Jo Moore, then a special adviser to former minister Stephen Byers, there has been increasing scrutiny of the role of political aides.

And as the government moves to put accusations of excessive spin behind it, the prime minister appears to have accepted that a more transparent role for advisers is necessary to re-establish public trust.

A new section to the code of conduct for special advisers will clarify relationships between special advisers and permanent civil servants.

"A strong, effective, politically impartial civil service is a great national asset," said Blair.

"In general, the relationships between ministers, special advisers, permanent civil servants work extremely well.

"But we agree with the committee that a number of further sensible, practical steps can be taken, building on the work of the Phillis Review published last week, which will help us work together to deliver our ambitious programme of reform."

The government also recommitted itself to a Civil Service Act and agreed that in future the appointment of the First Civil Service Commissioner will be made following consultation with the leaders of the main opposition parties.

Responding to the announcement, civil service union the FDA said it was "extremely concerned at the continuing and insidious campaign to allow ministers more say in the appointment and promotion of civil servants".

"We have no doubt whatsoever that such a facility would be abused," warned general secretary Jonathan Baume.