Labour would set up a national register of qualified heads, David Blunkett, the party's education spokesman, promised this week.
Potential headteachers who gained the proposed new national qualification would have their names added to the register, which would be available to governors.
"After a reasonable period, to enable the scheme to develop, governors would expect all new heads to have successfully completed the new course that would give them a headteacher qualification," Mr Blunkett wrote in The Independent.
Labour's education spokesman said he and Tony Blair were "delighted" that Gillian Shephard, the Education Secretary, had adopted their plan for a new headteacher qualification. His suggestion of a national register is an obvious attempt to regain the initiative.
John Sutton, general secretary of the Secondary Heads Association, dismissed the idea of a register as "unreal and unnecessary". If it was an attempt to differentiate Labour policy from that of the Government, it was meaningless.
"The qualification shouldn't be compulsory," he said. "If the system of qualification and accreditation acquires street credibility, then it will follow that anyone who hasn't got it won't stand a cat in hell's chance of getting a job."
The National Association of Head Teachers is also wary of the idea of a compulsory "certificate of airworthiness". David Hart, its general secretary, said: "We're absolutely behind any initiative - from Gillian Shephard or David Blunkett - to provide a rigorous qualification for those aspiring to be heads. But if either of them says it should be compulsory, they must guarantee equality of access or it's a non-starter. And we still have reservations about whether a certificate can prove that anyone will be an effective leader. "
Labour's proposals are understood to be based on practice in the United States, where each state licences headteachers, who are all expected to have Masters degrees. The licence used to be open-ended but now has to be renewed after three or five years.
Labour does not envisage a time limit. Once heads had gained their qualification, they would stay on the register unless they were, for instance, guilty of gross misconduct.
Labour would use a modular approach to train potential heads, Mr Blunkett said.
In fact, the qualification now being developed by the Teacher Training Agency, at Mrs Shephard's request, is likely to be modular in any case. It will be equivalent to a national vocational qualification at level 5 and pilot schemes will operate from the autumn of 1996. Candidates will study for the certificate using distance-learning methods.