Ken Livingstone's green initiative is one of the targets for a German energy company. The mayor of London is committed to buses powered by hydrogen fuel cells, a technology developed by Proton Power Systems, which lists on the Alternative Investment Market on Wednesday.

The company developing the fuel cells, which generate electricity, is in talks with a UK bus manufacturer. Felix Heidelberg, Proton's chief executive, said: "London is an attractive market because of its commitment to fuel cells."

Transport for London is testing three hydrogen fuel cell buses and has issued a tender for 10.