A review of a policy to give under-16s free bus travel is being called for after it was revealed that it will be open to children from across the UK.
The free travel scheme hopes to boost bus use
London Assembly Conservatives say Londoners should not be subsidising children who are just visiting.
Mayor Ken Livingstone admitted the scheme was open to all under-16s regardless of where they live.
But he said it was unlikely children outside London will fill out forms for something they will hardly use.
From last month, under-16s can travel for free on London buses and trams, although those aged 14 and above will need to carry a children's travel card.
The scheme was originally designed for children living in London but in response to Conservative's written questions, the mayor admitted it was open to all children.
Roger Evans, Conservative member and chairman of the London Assembly Transport Committee, said: "This is one big freebie for visitors at the expense of Londoners.
"It's a disgrace. We've always said this scheme was an unaffordable luxury - and here's the proof."
But Mr Livingstone said: "If the Conservative members of the London Assembly want to get rid of free travel for children under 16 they will have to put it in their manifesto and win a mayoral election.
"Meanwhile, I was elected mayor on a manifesto commitment to this and 80 per cent of Londoners support the policy.
"To qualify for free travel 14 and 15-year-olds have to have an Oyster photocard. Some children just outside the London border travel to school in London.
"Apart from that, the number of children outside London who will go to the trouble of coming into London and getting the forms for something they would hardly ever use, is tiny and will have no appreciable cost to London."