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"Avaya has entered a consultation process following the strategicbusiness decision to potentially close the Monkstown office," it said in a statement.
Local representatives at trade union Unite have accused Avaya and acting Northern Ireland first minister Arlene Foster of leading Nortel staff "up the garden path" over their futures.
As reported by MicroScope, the Monkstown, Belfast plant saw 87 jobs axed last March and at one stage was the subject of a question raised at Westminster over the future of Nortel workers' pensions.
At the time Nortel told MicroScope that it remained committed to the future of the plant as "the centre of its EMEA supply chain operation" but in the wake of the $915m acquisition of Nortel Enterprise by Avaya late last year, this outlook has clearly changed.
The plant was formerly owned by Standard Telephones and Cables (STC plc), which pioneered digital phone and optical fibre technology, but overspent trying to break into the mainframe business in the 1980s, and was subsequently bought by Nortel in 1991.