Confidential talks have taken place with the privately-owned Dutch postal service that could result in it sorting and carrying millions of letters posted in Britain to addresses abroad.
A senior manager disclosed that under the plan the Dutch service, now run by the courier company TNT, would handle a proportion of international mail at its own depot.
The proposal was denounced as "backdoor privatisation" by furious union leaders who are to seek an urgent meeting with Post Office directors and will press Stephen Byers, the trade and industry secretary, to halt the scheme.
Details emerged at a sensitive time for the Post Office. It is due to report its first loss for almost 25 years later today. It is also in delicate negotiations over plans to shift 1,500 jobs out of central London's Mount Pleasant sorting office and seek union agreement on staffing a new international complex to open next year at Langley near Heathrow.
Union leaders threatened industrial action to stop a deal that would for the first time give a private firm responsibility for Royal Mail post.
John Keggie, CWU deputy general secretary, said: "We will not allow work to be handed to a competitor. We recognise commercial freedom has put us in a different environment but that does not mean handing over the family silver to a rival."
Discussions between John Mod, managing director of Royal Mail's international division, and the Dutch post office are understood to be at an advanced stage;one insider claimed a draft agreement had been signed.
The British, Dutch and Singapore post offices announced an alliance earlier this year to win new business around the world in the face of fierce German and US competition.
The Post Office yesterday insisted no deal had been signed to involve the Dutch company in the UK postal system.
But an industry source maintained that under the proposal the Dutch TNT Post Group would win a share of the letters leaving Britain once it had proved it could do so more cheaply than Langley.
The Post Office is expected to announce today a deficit of around £250m in the year to the end of March.
Industry sources blame the government's decision to pay state benefits directly into bank and building society accounts - rather than over post office counters - for the slide into the red.
The change is thought to have cost some £400m in revenue last year and meant that the Post Office had to write off £571m of a £1bn investment project for counter services automation.
Though the Post Office would have made money without the write-off, today's figures are expected to show a downward trend in its profits reflecting growing competition as well as the loss of revenue from benefit payments.
There are fears for the future of many post offices, especially those in rural and inner city areas. Only 600 post offices are directly run by the Post Office directly. The remainder are run by sub postmasters, most of whom own the businesses and are paid for the services they provide.
Some 8,000 are said to be making losses and are being subsidised by the Post Office.
