A union fears hundreds of jobs could go in a shake-up of mail services in Coventry and Northampton.
The firm is planning to replace mail centres in the two places with a new combined centre, likely to be built in Northamptonshire.
The Communication Workers' Union fears the existing Coventry operation will be badly hit and has warned of possible industrial action.
Royal Mail said any job losses would be through natural wastage.
The new mail centre, which is expected to be ready in 2009, will deal with all post coming from and going to Coventry and Northampton.
This will then be passed on to local sorting offices in the two places.
The change will mean the closure of the Royal Mail's Bishop Street mail centre and sorting office in Coventry.
Local sorting will be carried out at two new offices being built in the city.
The sorting office at Crow Lane in Northampton will be replaced and one in St James Mill Road refurbished.
Royal Mail's area general manager for Central and South Midlands, Michael Stockdale, said: "It is expected that with some flexibility everyone who wants a role in the new structure will have one.
"Until final detailed plans are completed it is not known how many people will be at each of the new locations."
Royal Mail said the mail centre was likely to be in Northamptonshire "due to the strategic significance of the area" to its distribution network.
Mick Kavanagh, from the union, said: "Our members are shocked. They've kept it away from the union.
"On the one hand they tell us there'll be a job for everyone.
"On the other hand they've broken a national agreement by converting full-time jobs to part-time across the Midlands.
"It's good news for Northampton. But the difficulty is how many jobs will there be? New technology might make people redundant."
It has not been decided exactly where the new units will be and the plans are still subject to final approval.
The new local sorting offices are expected to open in 2008.
One of them is likely to be near Coventry city centre, with the other being to the south-west of the city.