Boris Johnson last night said he could "absolutely guarantee" that Tramlink would be extended to Crystal Palace by 2020.
Speaking at a Conservative Party fundraiser, the Mayor of London said the much sought-after extension has been approved by Transport for London commissioner Peter Hendy.
But he added a precise timeframe for the extension was yet to be established.
Mr Johnson said: "It is a very slow-moving process but it is going to happen.
"It may not be by 2015 but it is going to be by the end of the decade. I can absolutely guarantee that."
Mr Johnson and TfL has come under repeated fire in the past for failing to allocate funds for the extension despite publicly backing it.