Ken Livingstone has denied he was planning to extend the congestion charge to other parts of London.
Reports said the mayor had "secret" plans to extend the scheme if he is reelected in May.
A source close to Mr Livingstone claimed the plan was hushed-up - and deliberately excluded from his transport manifesto unveiled yesterday - under strict instructions to avoid losing votes in the forthcoming election. The manifesto contains no new commitments to congestion charging but confirms a £25-a-day toll on bigger engined cars will come into force in October.
But today a spokesman from the mayor's office said the rumours were "absolutely not true". He said he knew of no feasibility studies into extending the congestion charge. Today former government - and TfL - advisor, Professor David Begg, said such a scheme would be politically risky.
But by extending the charge to the M25 the scheme could raise £2 billion a year and cost £1 billion a year to run.
That would provide more than enough revenue to totally overhaul London's transport system.
He added: "I think it would be right for London. It would bring important reductions in pollution and congestion. It would raise substantial amounts of revenue. It could be used to reduce council tax and that could be very attractive to the boroughs."
Car manufacturer Porsche today said it would take legal action over City Hall plans for a £25 "gas-guzzler" congestion charge. Mr Livingstone said he was "prepared to fight Porsche in the courts".
