Friends of the Earth has issued a challenge to London's newly-elected Mayor, Boris Johnson, to outline how he plans to live up to his commitment to make London the "greenest city in the world".

Speaking after it emerged this week that Johnson remains committed to his manifesto pledge to scrap the planned £25 a day congestion charge for the most polluting vehicles, a spokesman for the lobby group said that the onus was now on the new mayor to explain how he planned to curb carbon emissions.

"Friends of the Earth supported the changes to the congestion charge, because it would have helped cut emissions and it would have encouraged people to buy greener cars – which is why Porsche was opposing the move," he said. "If the Mayor is going to scrap these changes he needs to spell out how he plans to tackle climate change and cut carbon emissions."

He added that the first few months of Johnson's term of office represented a "critical" period for the Tories, because it will allow observers to gauge how serious the party is about its recent attempts to position the party as a greener alternative to the government.

Johnson has maintained that the proposed £25 charge would not deliver significant reductions in carbon emissions and that there are more cost effective means of reducing London's carbon footprint. He said earlier this week that he would back up his manifesto pledge to scrap planned changes to the congestion charge, insisting there "will be no £25 charge for large cars, which would hit the wallets of families and small businesses hardest".