Mayor Ken Livingstone's scheme to charge drivers £5 a day to enter central London between 0700 GMT and 1830 GMT begins on 17 February.

He hopes it will reduce traffic by 10% to 15% and raise at least £130m a year towards improving public transport.

But it may need to be improved and extended if congestion across London is to be reduced, said David Begg, who chairs the government's Commission for Integrated Transport.

Could Heathrow get its own charge zone?

Mr Begg, who is also on the Transport for London board, told the Financial Times: "There's a big growth in traffic and congestion in outer London and we need to look at tackling that.

"It can't be tackled just by improving public transport.

He added: "If (the scheme is) successful then Transport for London needs to look at the zone where the cordon is, the level of the charge and more sophisticated electronic technology which allows you to distinguish between peak and off-peak travel."

People living on the boundary fear it will increase congestion on the inner ring road, which is itself not subject to the charge.

But Transport for London has said there are no plans to extend the charging area yet.

A spokesman said: "This is something that the mayor has talked about in the past but only once the charge is in and has been assessed.

"The mayor has talked also about a charging zone around Heathrow, for example."

Any extension would not take place before the next mayoral election in 2004, he added.