The independence of the civil service is under threat from special advisors, a former cabinet secretary has warned.

Writing in the Spectator magazine, Lord Armstrong of Ilminster, who was head of the civil service between 1983 and 1987, called for a reduction in the number of special advisors.

Their role needs to be more clearly defined to prevent a politicisation of the service, he argues.

"Recent events have made it increasingly clear that the time has come to limit the number of special advisor, and to define with unmistakable clarity the duties and responsibilities that are proper to special advisors and those which are not, and their proper relationship with ministers and with civil servants," he writes.

His intervention follows the row over that engulfed Stephen Byers and led to the resignations of the transport department's chief press officer, Martin Sixsmith, and special advisor Jo Moore.

Lord Armstrong says that the non-political professional career in the civil service is under threat, and that action needed to be taken.

"As we are seeing, if they do not do this, they face, quite apart from the corrosive effect on public administration, the increasing risk of running into a succession of political crises which will endanger their own positions."

Should such erosion occur, public mistrust of government would only increase, warns Lord Armstrong.

"Special advisors are particularly concerned with considerations of political spin that did not enter into the business of news management; but it seems to me that the balance has now swung too far in that direction," he said.

"What the practitioners of this black art sometimes fail to recognise is that the main casualty is their own credibility; people are increasingly reluctant to accept what they put out as gospel truth."