In highly sensitive confidential talks, senior Labour politicians have told leaders of the employees' group at GCHQ - denounced as "scabs" by trade unionists - that there will be no automatic right of reinstatement for orthodox unions following their historic ban 12 years ago.
The revelation will lead to allegations that Tony Blair is reneging on his much-repeated commitment to "full union rights" at the intelligence listening post - a cause celebre in the labour movement.
In private talks, Shadow Cabinet members David Blunkett and Michael Meacher have informed leaders of the Government Communications Staff Federation (GCSF) that a Labour government would negotiate with them if they could prove they represented half the GCHQ employees.
Other civil service unions, now with no members at the centre, would only be considered for recognition if they could recruit substantial membership.
The staff federation estimates it already represents half the workforce, but the government-appointed Certification Officer has so far refused an official seal of independence. Representatives of the association believe they will gain the crucial certificate this month.
Last December, the director of GCHQ relinquished his veto over the organisation's decisions following pressure from the UN-backed International Labour Organisation (ILO). The decision by the director paves the way for official certification.
The removal of the GCHQ director's veto has enabled the Government to escape condemnation by the ILO this summer. In parallel with the talks with members of the Shadow Cabinet, the staff association has also privately approached the Whitehall unions which were banned from GCHQ.
Brian Moore, chairman of GCSF, has suggested that one way of avoiding a scramble for members at the complex under a Blair government would be if his organisation entered a federal structure with one of the Whitehall unions.
Mrs Thatcher banned unions because she believed that they were a security risk, but a strictly federal system would allow GCHQ staff separate negotiating arrangements.
Mr Moore said that his organisation did not proscribe industrial action, but hoped that such tactics would never be necessary. He hoped that GCHQ management would think again about its refusal to allow staff the right to go to an industrial tribunal.
Mike Grindley, leader of the GCHQ employees who were dismissed for refusing to give up union membership and regarded as something of a folk hero in the union movement, said he would oppose any attempt to give the GCSF sole recognition. He denounced the abolition of the GCHQ director's veto as a "cosmetic" and argued that industrial action was still effectively banned at the centre.
Officials of the staff association were full-time staff at the intelligence centre and therefore could not claim to be fully independent. He hoped national union officials and the Labour Party would not be "taken in".
Labour leaders argue that there is no question of abandoning the pledge to allow GCHQ employees the right to join the union of their choice. Under the present plans - and contrary to the strong impression given to TUC affiliated unions in the past - there would be no automatic recognition for such organisations.