Congestion in central London is almost as bad as it was before the daily charge was introduced four years ago, according to official figures.

Traffic delays have risen sharply in the past two years and will rise further next week when the zone doubles in size with a westwards extension into Kensington and Chelsea, Transport for London said.

The loss of most of the benefits of congestion charging is causing concern in other cities that have been considering whether to follow London’s lead.

Ken Livingstone, the Mayor of London, is so concerned by the rise in delays that he is planning to bring forward the introduction of a £25 daily charge for vehicles with high emissions from 2010 to June or July 2008. Almost a fifth of vehicles (18 per cent) that currently pay the £8 charge will be liable for the new top rate, which applies to cars in band G for road tax.

Mr Livingstone said yesterday that people who drove to work in a band G car would pay £6,000 a year under the new rate, with no discount for people living inside the enlarged zone. He said that he hoped that they would “take the opportunity to switch to public transport”.

Congestion fell by 30 per cent in the first year of the charging scheme but is now only 8 per cent below precharging levels. TfL predicts that congestion will rise by 4-5 per cent on current levels from next Monday because 60,000 car owners living within the extension area will gain a 90 per cent discount on the charge when entering the existing zone.

The charging period will also end at 6pm instead of 6.30pm, meaning that the queues at entry points, which build up just before the end of charging, will start half an hour earlier.

The number of vehicles entering the zone fell slightly last year and is now 20 per cent below the level in 2002. The fall in vehicles has failed to produce a lasting reduction in congestion because the capacity of the road network has fallen over the past four years.

Mr Livingstone said that utility companies such as Thames Water were to blame for most of the rise in congestion because they were digging up more roads and taking longer to reinstate them.

He published figures showing that the duration of works by all utilities tripled between 2004 and 2006, largely due to the replacement of leaking Victorian water mains.

The Mayor admitted that the introduction of new bus lanes and sets of traffic lights had also reduced the capacity of the network, although he claimed that the effects of these were small compared with the delays caused by road works.

He claimed that without congestion charging central London would regularly be gridlocked.

But the RAC Foundation said that Mr Livingstone had failed to take into account that congestion was partly self-regulating, with a proportion of people choosing not to drive once average delays reached a certain level.

Edmund King, the foundation’s director, said: “Traffic speeds wouldn’t be much different without congestion charging because drivers would switch if delays got too long.”

Steve Norris, who has consistently argued that congestion charging is a waste of money and was beaten twice by Mr Livingstone in mayoral elections, said: “In politics, I do know that saying ‘I told you so’ is deeply unfashionable. But what the hell. I told you so.”