London's mayor has been accused of using the congestion charge to "boost his coffers" after announcing plans to raise it to £10 a day by 2008.
The charge rose to £8 last July
In a televised debate, Ken Livingstone said the £2 rise could be included in his next manifesto, and any extra money would go back into public transport.
But his political rivals and critics of the daily charge have accused him of using it to raise money.
It was £5 when it was introduced in February 2003, rising to £8 last July.
In February 2007 the boundary of the congestion charging zone will be extended westwards to include Kensington and Chelsea.
Among those lining up to criticise plans to raise the charge were the AA Motoring Trust, who said any further reduction in congestion would be "marginal" and the Forum of Private Business (FPB).
FPB chief executive Nick Goulding said: "It is now clear that it is being used as a tax to milk motorists and small businesses."
Angie Bray, London Assembly Conservative transport spokesman, said: "Whenever he needs more money the congestion charge and bus fares are where he turns."
And the transport committee's Liberal Democrat spokesman Geoff Pope added: "He appears to be doing this just to boost his coffers."
When the charge - payable by motorists driving into central London - was introduced it was thought congestion fell by about a third.
When the last rise was brought in in July, it fell a further 5%-6%. It is thought another rise will bring congestion down further, which the mayor has said is vital to keep it under control.
Mr Livingstone also confirmed that bus fares would rise by 10% next year in order to help repay the £3bn government loan for public transport.
He revealed that 400 pupils, out of 300,000 under-16s who had taken up the offer of free travel, have had their bus passes withdrawn due to bad behaviour