An historic mail train service which runs under London faces closure after 75 years, unless a new backer can be found to save it.

Royal Mail says the Mail Rail service which runs from west to east London is no longer proving viable.

In its heyday the "unique" service, the only one of its kind in England, served nine stations, carrying four million letters per day.

Royal Mail says it recognises the heritage value of the service

But a massive drop in postage in the city, plus changes in how mail is distributed, mean the service has become too expensive, say its owners.

With just four stations currently in operation on the route and the likelihood of only three by next year, the Royal Mail is seeking help from private and heritage groups to continue the service.

David Chapman, Royal Mail's London programme manager, said it was sensible to consider the future of Mail Rail given the financial losses suffered by the postal business.

'Not sustainable'

He said: "It serves fewer stations than originally intended and it costs us five times as much as moving mail by road.

"For a business losing �1.2m a day, that is clearly not sustainable."

The idea of an underground rail service for London was first mooted in 1855 by the then secretary to the Post Office, Rowland Hill.

Trains travel at 40mph in tunnels separate to The Tube

Although trials were conducted in small tunnels, the Post Office abandoned the scheme until early in the next century. It was eventually given the go-ahead by the government in 1913.

The outbreak of World War One halted construction and the little tunnels were used to store art treasures from major London galleries, such as the Tate and the National Portrait Galleries.

Work began on the tunnels again in 1923 and the first trains started delivery four years later - under the banner of the Post Office Underground Railway.

It became Mail Rail on the service's 60th anniversary, shortly after the old stock was replaced with 34 new trains.

Automated trains

Today Mail Rail runs along a 37km route between Paddington in west London and Whitechapel in the East End, and is staffed by 76 postal workers.

Royal Mail wants to hear suggestions and ideas from businesses, heritage and other groups about what should happen to the automated trains, tracks and tunnels.

Mr Chapman said the service was now "well past its prime" but he recognised the historical and heritage value of the underground railway, which is totally separate from London Underground's Tube network.

A spokesman for Royal Mail said the company was carrying out a review of mail posting in London.

Mail Rail Facts

Trains are 8.4m long They carry loads of 980kg of mail The tunnels are 21m (70ft) underground Trains run on a 610mm electrified track

He said: "This will take about five to six weeks at the end of which we will take a decision on Mail Rail's future," he said.

The amount of mail posted in London over the past five years has dropped by virtually the equivalent of a mail centre and volumes continue to decline because of the economic climate and increased use of email.

The Communication Workers Union said it was concerned about any job implications of Friday's announcement.

The Royal Mail spokesman said in the event of any closure, staff at Mail Rail would be treated in accordance with its policies on surplus staff.