London mayor Ken Livingstone has said he will make a decision on whether to go ahead with the 100% discount for cars emitting 120g/km or less of CO2 in the congestion charge zone before Christmas.

However, fleets are being warned that the green light for the discount is still not certain.

Rumours in the industry have suggested that, due to the increasing numbers of sub-120g/km cars that would qualify for the discount, including the Ford Focus, Peugeot 308 and Audi A3, the mayor may decide that the proposal is counter-productive to keeping cars out of the capital.

Fleet operators’ association ACFO called for a swift conclusion, saying that decision-makers needed clarity for their planning.

A spokesman for the mayor’s office said: “The consultation has finished and Transport for London is due to present its report soon.

“Then it will be up to the mayor to make a decision to go ahead, modify it, or not go ahead at all.”

The British Vehicle Rental and Leasing Association believes the proposal will go ahead any day now.

Spokesman Robin Mackonochie said: “The mayor’s record on giving projects which have gone out to consultation the go-ahead suggests this will go through. We would be surprised if it didn’t.”

If it does get the green light, the discount would take effect in April, ahead of the mayoral election on May 1.