10 million Oyster Cards have been issued

Ken Livingstone said the cards had revolutionised travel in London, speeding up buses and Tubes and reducing queues.

The free cards, which usually involve a deposit of �3, will be available to anyone who has not got an Oyster.

Currently there are 10 million card holders and about 75% of bus and Tube journeys are made using an Oyster card.

"This campaign is encouraging Oyster uptake amongst passengers who are still paying cash and therefore paying more," said Mr Livingstone.

The giveaway, advertised through national, local and community press adverts, will target London communities where Oyster take-up is low.

Currently you have to pay the deposit if you do not buy a monthly or annual travel card.