Drivers of high-polluting cars could face a congestion charge of £20 in London, the mayor has said.

The higher toll would include four-wheel drive vehicles and some high-end sports cars, which emit up to four times as much pollution as ordinary cars.

Mayor Ken Livingstone said the charge would not be based on car size, but on their carbon emissions.

"We've got a target to try and reduce carbon emissions by 20% in this city from the 1990 level. Now, we're three-quarters of the way there, but that last quarter is going to be the difficult one.

"Therefore, I would very much favour the idea perhaps of the £20 charge for cars that emitted two or three times the normal level of carbon emissions."

In a GMTV interview, Mr Livingstone admitted it would be hard to ensure the right cars are charged. "Identifying cars, making sure what car emissions are by make and model and year, it's a much more sophisticated system."

Transport for London has started work on seeing whether the C-charge system can be refined, he added.

"Whether we can get the system sophisticated enough, we won't know for some months, but we are certainly looking at it.

"If you could find a way of actually doing that... you encourage people in London when they next buy a car (to) buy one that pollutes less."

Currently all drivers have to pay £8 to drive into the C-charge zone in central London between 7am and 6.30pm on week days.

From Monday, drivers will have an extra day to pay the toll. Up until midnight on the day they enter the zone, the charge will be £8. If they pay the next day, it will be increased to £10.

At the moment motorists are fined £100 if they fail to pay on the day they drive through the zone.

Geoff Pope, chair of the London Assembly's transport committee, welcomed the new next-day payment. "Too many people were caught out because they simply forget to do it the same day," he said.

"But an even fairer system would allow drivers to pay the basic daily rate the following day."

London First, which represents about 300 of the capital's businesses, also approved of the move. It will reduce the "hassle factor" of the C-charge, chief executive Baroness Jo Valentine said.

C-charge users can pay by calling 0845 900 1234 or through the congestion charge website, www.cclondon.com.