A union representing hundreds of postal workers is opposing the Royal Mail's plan to close a sorting office in Slough, Berkshire.
Slough Mail Centre is due to close in March 2005, as part of the company's review of its mail centre network.
Royal Mail says the lease on the
building is due to expire and there is
insufficient room for new technology.
But the Communications Workers' Union (CWU) claims the company had "always intended to close the centre".
The changes will mean that mail collected from customers, pillar boxes and Post Office branches in the area will be sent to Royal Mail's £55m mail centre in Greenford for processing.
Royal Mail says mail deliveries will not be affected by the transfer of services because the Slough delivery office will remain at the Slough Mail Centre site - which is owned by the company.
More than 270 staff currently working at Slough Mail Centre will be affected by the closure proposal.
Employees will be offered the chance of transferring to Greenford Mail Centre or to Heathrow Worldwide Distribution Centre in nearby Langley, where 1,200 jobs are being created.
They will also be offered voluntary redundancy, a Royal Mail spokesman said.
Members of the CWU voted unanimously to oppose the closure plans on Sunday.
CWU branch chairman Charlie Badzek said: "Royal Mail's proposals would have a devastating impact on our members lives.
"It is not only our members that would suffer but the public would see the effects in poorer quality of service.
"We urge the public to join with us in opposing the closure"
A Royal Mail spokesman said: "The decision we have announced regarding Slough in no way reflects on the performance of the employees who work there.
"On the contrary, we are grateful for the contribution they have made to date and we will make sure each individual is dealt with fairly and sensitively as we move towards the closure of the centre."
Talks between CWU members an the Royal Mail are expected to take place at the Slough Mail Centre on Tuesday.