London's black cabs are set to turn green in a bid to clean up the capital's air - but Londoners will end up footing the bill.

Mayor Ken Livingstone has announced radical new regulations for the city's 20,000 cabs which will have to meet new emissions standards by 2007.

Mr Livingstone's Taxi Emissions Strategy claims cabs are responsible for 24 per cent of fine particle and 12 per cent of nitrogen oxide of road transport emissions in central London.

These pollutants can cause heart and lung diseases and can trigger respiratory problems among the vulnerable, including the growing number of Londoners who suffer from asthma. In central London these pollutants are present at levels that damage the health of many residents.

The Mayor's office said the cost of converting black cabs to reach the new standards will be met by a flat rate fare of 20 pence per journey from April 2005, over and above the usual cost of living rise. This fare rise will be reviewed after three years.

Mr Livingstone said: "London has the worst air quality in the UK and air pollution is estimated to cause 1,600 premature deaths every year in London. For just 20p a journey we will see major health benefits for all Londoners.

"Taxi drivers are exposed to more pollution because they spend long hours on the road each day and tend to operate in the most polluted areas, such as central London and Heathrow, and the cab trade will also therefore be a major beneficiary of this policy."

He added: "This is one of a number of measures I am putting in place to improve air quality in London. The central London congestion charge is already having an impact, with emissions in the zone cut by 12 per cent. I am also committed to making London a Low Emission Zone, which will see the removal of the most polluting lorries, buses and coaches from the streets."

Ed Thompson, Transport for London Taxi and Private Hire Director, said: "London already has the best taxi service in the world, and before long it will have the cleanest."

Lynne Featherstone, Chair of the London Assembly Transport Committee, said: "Taxis are expensive enough. Although tackling pollution is important for Londoners' health, I am seriously concerned that allowing fares to rise even further could lead to more people using unlicensed minicabs because they are considered the cheapest form of transport to get home on a late night.

"We all care about clean air, but safety is paramount.

"The Mayor keeps finding excuses to increase transport fares in London, yet we see only minor improvements to services. Black cabs are going to be priced out of the market, fewer passengers will be able to afford to use them and this could mean less taxis available outside of central London."