The satellite is scheduled for launch in 1995 and will carry telecommunications traffic until the end of the century, transmitting calls from portable telephones.
A Russian Proton rocket, first built in 1965 and one of the most reliable launchers of the old Soviet Union, will take the satellite into orbit 36,000 kilometres (22,370 miles) from Earth.
The deal, between the International Maritime Satellite Organisation and DB Salyut, the designers of the Proton, means that it will be possible to make telephone calls via satellite from the remotest regions of the world only thanks to 30-year-old rocket technology.
Inmarsat, which has its headquarters in London, said it is paying DB Salyut dollars 36m ( pounds 24m) - about half the price of using American or European rockets - but admitted that there would be additional costs, yet to be assessed, 'because of the pioneering nature of the launch'.
Olof Lundberg, director general of Inmarsat, said that although the launch costs were expected to be less than in the West, the risk and uncertainties were greater.
Only three out of the past 100 launches of the Proton rocket have ended in failure, according to Dmitrii Polukhin, director general of DB Salyut, who was in London to sign the contract.