The filters are designed to reduce harmful emissions

Some 1,000 out of 16,000 cabs have so far been fitted with the filters, which cost �2,000 each.

The London Cab Drivers Club claims its own tests on four cabs showed the filters made the air dirtier.

Transport for London (TfL) rubbished their claims which they said were based on "dubious test results of a very small number of vehicles".

Green pledge

The London Cab Drivers Club (LCDC) said tests on the four different types of filters they were required to install showed that even the best performing filter emitted 30% more pollution.

"They are doing more harm than good," said Mr Fleming, of the LCDC - a black cab trade association with 900 members.

The �32m cost of the scheme is being funded by passengers through a 20p taxi fare surcharge.

London's 16,000 black cabs need to be fitted with the filters by June 2008 as part of the mayor's pledge to provide cleaner transport.

TfL said it was confident that the filters, which have been approved by the Energy Savings Trust, met with strict European emissions standards.

"It is completely irresponsible to claim that exhaust filters being fitted to London's black cabs are increasing toxic emissions, based solely on the dubious test results of a very small number of vehicles."