Earlier this week the company announced a £70million investment in the new centre, which will be developed at junction 15a of the M1 on the Swan Valley industrial estate near Northampton.

The site will replace the temporary sorting office in Crow Lane, as well as ageing depots in Coventry, and the investment will also include a makeover at the St James Mill Road delivery office.

But yesterday a Royal Mail spokesman said the company was also considering using the new building to house operations from Milton Keynes.

He said: "As announced in 2007, over the next few years the company will be making huge investments in the network, including new technology and equipment for mail centres.

"The Milton Keynes Mail Centre site, as it stands, may not be suitable for the new technology and equipment needed because of its limited size, and as a result we are looking at a number of options for its future.

"These options include the possibility of transferring processing operations to the proposed new modern state-of-the-art mail centre on the M1. We are consulting with our people and unions and will make a decision as soon as possible."

Lee Barron, regional secretary for the CWU union in the East Midlands, said while he welcomed the long-awaited announcement for a new depot, union members would not want to swallow up Milton Keynes' operations.

He said: "I am delighted they have confirmed details about the new depot but we don't want it at the expense of anybody else. When the Billing Road depot burned down in 2003, our mail was sent out all round the country and we were worried they would close us down.

"We met with the people from Coventry and Milton Keynes and we had an agreement that we would not accept anybody else's work at the expense of anybody else.

"We will stand by that and make sure they protect their own workers."