Ministers should be blocked from appointing their own press officers, the Civil Service Commissioners have said.

In its annual report - published on Monday alongside evidence to the Phillis Review of the Government Information and Communication Service (GICS) - the watchdog warned that the rise of the hand-picked press spokesman could call into question the tradition of civil service impartiality.

The warning comes amid continued claims that the government is heading down the "slippery slope" towards the politicisation of Whitehall.

As Commons leader, Robin Cook came under fire after he appointed parliamentary journalist Des McCartan to the position of press spokesman.

Several top level government press officers have quit the civil service in protest at the creeping politicisation of the GICS.

The actions of Downing Street communications chief Alastair Campbell have also called into question the lines of accountability between full time civil servants and special advisers, who are essentially political appointees.

The rise of the political spokesman, warned first commissioner Baroness Usha Prashar, risks further undermining public confidence in government.

"My concern is that people trust the Government Information and Communications Service and that there is objective information being put out," she told the BBC.

And she said that special advisers should refrain from secretly briefing the press, warning that "the boundaries are not clear".

While she conceded that ministers would be involved in selecting press officers, she counselled against giving them total say over who acts on their behalf.

"Therefore, there should be proper involvement by ministers, but they should not be able to choose, because if they begin to choose the press officers, then of course it is a slippery road because it does seem that you are beginning to erode the impartiality of the civil service," said Baroness Prashar.

"At the same time, special advisers, if they wish to speak to the press, should do so on the record."

The commissioner insisted that the Civil Service Code should remain as the cornerstone of officials' conduct.

"At a time of rapid change there is a need, more than ever, to make sure that the values laid out in the Civil Service Code - integrity, honesty, impartiality and objectivity - and appointment on merit are not eroded," she said.

"As departments recruit more people from outside to senior positions, it is essential not only to make sure the civil service benefits from their skills, experience and different ways of doing things, but also that they understand the core values which underpin the work of the service."

Senior civil service union the FDA welcomed the report and said that it strengthened the case for a Civil Service Act to reinforce safeguards.

"The government has repeatedly committed itself to supporting a Civil Service Act," general secretary Jonathan Baume said.

"It is time that [they] stopped their equivocation and took positive steps towards legislation."