Plans to expand the central London Cycle Hire scheme have been criticised by a sustainable transport charity.
Sustrans said plans to enlarge the scheme did not include the outer London boroughs where there was a greater potential for cycling.
A Transport for London (TfL) spokesman said it could extend the scheme by adding more docking stations to the current central London area.
TfL also said it could move eastwards for the 2012 Olympic Games.
But Sustrans said funds should be spent on expanding potential for cycling in outer London.
"Plans to boost cycling in 13 outer boroughs appear to have been put to one side in favour of the high-profile and city centre cycle hire," a spokesman for the charity said.
"Assigning money to the boroughs would mean that each borough could spend their share on the cycling improvements that would have the biggest impact locally."
About 5,000 cycles are available across 330 locations in central London as part of the Barclays Cycle Hire scheme, which was launched on 30 July.
Kulveer Ranger, the London mayor's transport advisor, said Boris Johnson "has always said that if the scheme was a success he would be keen to see it expanded".
"However, this would be dependent on working with relevant boroughs and identifying suitable funding.
"Given the huge positive response to the scheme so far we are expecting other London boroughs to be beating a path to the Mayor's door to get cycle hire in their areas too."
A TfL spokeswoman said: "The mayor considers the Barclays Cycle Hire scheme to be the first phase of an expanding programme.
"Expansion could include extending the scheme to the east of London in time for the Olympics and adding to the number of docking stations in the existing cycle hire area."
TfL said it could not provide an estimate of how much the second phase of the cycle hire scheme would cost.