The cards are accepted on tube trains, buses, trams and the Docklands Light Railway network in London. TfL says around three quarters of all underground and bus payments in London are now by Oyster card.

But problems still exist for customers that use surface rail services to travel in and around the city.

Mayor of London Ken Livingstone wants the Oyster system to cover the whole of London's transport network, but he faces an uphill struggle to get commuter rail operators to implement the system.

Last May train operators were offered a £20 million package to install Oyster equipment at rail stations in an around London. They have until the end of January to accept the offer, but according to press report, most are poised to reject the proposal due to concerns about costs.

Says Livingstone: "Oyster has already proved itself to many millions of Londoners and it seems amazing that these companies haven't yet agreed to a deal which would meet their capital expenditure and would save their customers time, money and inconvenience."

Despite this Livingstone says "particular progress" is being made by some towards selling Oyster products outside of London.

TfL has also experienced delays and difficulties in extending the Oyster card to include functionality for low-value cashless payments.

The company said in March 2005 that it was looking to add payments functions to Oyster to enable customers to use the contactless card to pay for small purchases at retail stores. But this initial programme fell through after the company failed to agree technical and financial terms with potential partners.

Last month Barclaycard said its customers in London will soon be able to use their credit and debit cards to make contact-free, cashless payments and to travel around the capital under a deal signed with TranSys, the company behind the Oyster smart card.