The new contract is for a shortened seven-year cycle and allows the winner to place newspapers in the 250 tube and 20 bus stations across the TFL region between 6am and 11.30am Monday to Friday.
A tender of initial "expressions of interest" went up on the Official Journal of the European Union yesterday.
The contract only includes a morning edition, despite speculation that an afternoon copy would be added, or a second contract could be tendered for an afternoon freesheet to be distributed inside stations.
Associated Newspapers, publisher of the Metro, currently holds the 11-year contract, which expires in March 2010.
Potential bidders have until 3 June 2009 to respond in the initial phase.
Initially the contract was expected to be a straight shootout between Associated Newspapers and Rupert Murdoch's rival publisher News International (NI). It is widely believed that NI launched theLondonpaper as a prelude to launching its own morning title.
There are other possible contenders; Express Newspapers battled with Associated for the contract 10 years ago and it is believed that owner Richard Desmond could be tempted to try again.
Elsewhere, the sale of the Evening Standard to Alexander Lebedev in January has led to speculation that the Russian billionaire, who seems intent on causing a stir in London's media arena, could also go after the tender.
The success of the Metro newspaper, which has been turning over increasing profit after initially making a loss, could also entice bidders from abroad.
Associated Newspapers has maintained that losing the contract would not hurt the title, with it following the London Lite into the hands of outside vendors.
Richard Parry, interim managing director of London Underground, said: "The free morning newspaper is part of the fabric of city life and a success story for the publisher and our passengers alike.
"Londoners tell us it makes their daily tube and bus journeys better and the contract has generated millions of pounds which have gone straight into funding transport improvements across the capital.
"We believe that the new free morning newspaper contract remains a unique opportunity to bring value to more than 1bn journeys in London each year."
Transport for London launches coveted tender for freesheet distribution
Transport for London (TFL) has launched a tender process for its morning freesheet slot, which could see The Metro ousted from tube stations for the first time in 11 years.
