The zero-emission buses will now operate on Saturdays during the summer, as well as on weekdays, because their reliability and performance has impressed TfL officials so much.

Trialled as part of a European project to test new technologies in urban centres, the fuel cell buses were originally intended to run until December 2005, but their service has now been extended to around January 2007.

Nine cities across Europe are participating in initiatives to introduce more environmentally-friendly transport that reduces air and noise pollution, which has also involved route 25, which passes through the heart of London.

The only emission from the fuel cell buses being trialled is water.

Every weekday in the capital, more than 6,800 buses of all types transport six million passengers on over 700 scheduled routes.

Platinum catalysts are used in hydrogen fuel technology to promote the gases reaction with oxygen which produces fuel.