Deputy Secretary for Civil Service Michael Stone said those barred were likely to have to justify government policies in public.

He said civil servants who opted to join the committee should not make comments in conflict 'with significant policy, or their jobs'.

They would not have to abstain from voting for the provisional legislature, which the Government believes illegal, but would have to keep a 'low profile'.

A total of 28,000 officials will be barred.

Senior Non-Expatriate Officers' Association chairman Hui Kwok-hung said the guidelines were too rigid.

They might prevent civil servants taking part in the election of subsequent chief executives, he said.

Under the Basic Law, the SAR chief will be returned by an election committee until at least 2007.

Mr Hui said his union would consult legal advisers to see whether there was a case for a judicial review.

He cited Clause 21 of the Bill of Rights, which says the right to participate in political activities, to vote or to be elected, should be equal among permanent residents.

Mr Hui added that only officers ranked grade D3 - deputy secretary or deputy head of department - or above should be restricted.

'I can't see any conflict of interest in choosing the chief executive,' Mr Hui said.

Under the guidelines, Dr Leung Chi-chiu, nominated by the Medical Association on Monday, will have to abandon his bid.

Dr Leung, a consultant doctor in the Department of Health, said he would consult his staff union and the Medical Association before making a decision.

Preparatory Committee member Professor Lau Siu-kai said the guidelines would increase friction between Government and civil servants.

Nominations open tomorrow for the committee, which will have 400 members.