Operators of lorries, buses and coaches that fail to meet low emissions targets are being charged £200 (€265) a day to drive into London from today.
London Mayor Ken Livingstone's controversial Low Emission Zone (LEZ) is aimed at cutting harmful pollution in the capital.
Covering most of Greater London - unlike the £8 daily congestion charge, which only applies in the centre - it will target the most polluting older heavy vehicles.
Anyone who fails to pay the LEZ charge will be fined £1,000, reduced to £500 if settled within a fortnight.
Transport for London has warned that emissions criteria will become more stringent in the future, with even some ambulances affected from July.
Environmentalists and health campaigners support the scheme but business leaders are furious. The Road Haulage Association has estimated that nearly 20,000 goods vehicles will be affected and warned that the move could push up council tax and the cost of living.
Mr Livingstone said the £200 charge was designed to encourage operators to clean up their fleets rather than incur the charge.