The Mayor of London is under pressure from carmaker Volvo to make public the findings of a review into congestion charge exemptions.

Volvo Car UK has written to Boris Johnson today calling for him to stick to his promise to deliver the findings before the end of 2009.

It follows the carmaker's participation in successful lobbying of Johnson in June, when it asked him to review congestion charge exemptions which don't apply to conventional cars with lower emissions than some hybrid cars which are exempt.

In essence, under current legislation, the driver of a hybrid vehicle can travel within the Congestion Charge zone free-of-charge while the driver of a similar, or even lower, CO2 emitting conventional internal combustion-powered car is charged £8.00.

Volvo Car UK believed that this disparity was unfair and called upon The Mayor to review it.

In August 2009, following pressure created by Volvo and the ensuing media reporting and online campaign, Johnson agreed to review the exemptions. At the time, he confirmed that he would publish his findings by the end of the year.