South West Trains (SWT) passengers will be the last to be able to pay for their rail travel using their Oyster cards.

Mayor of London Boris Johnson is expected to make the announcement next week that Oyster cards will be accepted to pay for single journeys on most suburban trains by September.

But while Oyster agreements with most train companies are near, negotiations with SWT, who run a number of train lines from Waterloo, are not as advanced.

Complex franchise negotiations between the Department for Transport (DfT), SWT, and Transport For London (TfL) mean it is unclear when a deal will be struck which sees passengers able to use their Oyster cards to pay for one-off journeys on SWT trains.

The sticking point of the deal is over pricing, how much it will cost SWT to use the Oyster system, and whether the DfT still intend to roll out a national Oyster style smartcard system themselves.

Transport for London are now installing Oyster machines at all stations across London, to the cost of the tax payer of £40 million, in anticipation of the Londonwide rollout of the pay-as-you-go system.

Oyster card readers are in use at many train stations in the capital, including SWT stations, but passengers can only use them currently if they have a season ticket loaded onto their Oyster card.

Mr Johnson said it was “absolutely crackers” that rail passengers still had to buy paper tickets.

A spokeswoman for London travel watchdog Travelwatch said the organisation was hopeful the Mayor’s words meant Oyster cards would be implemented on the rail network soon.

She said: “We are encouraged but we have heard these kind of announcements countless times before so we will not be putting our mortgages on it happening when the mayor says.”

She added that it was in passengers’ interest for SWT to finish their negotiations and except the Oyster system as soon as possible.

Stations that use South West Trains include Clapham Junction, Earlsfield, Putney, Wimbledon, Raynes Park, Kingston, Surbiton, New Malden, Epsom, Esher, Weybridge and Walton-on-Thames.