An anti-poverty charity has launched a campaign against the use of biofuel, which is targeted at Transport Secretary Philip Hammond.
ActionAid has covered 50 buses on routes 507 and 521, destined to stop outside Mr Hammond's office, with adverts containing the slogan 'Biofuels. What is the human cost?'.
The advertising campaign is part of the charity's bid to urge the transport secretary to consider the impact that biofuel has on developing countries, as the push for land is leaving some of the world's poorest people homeless and hungry.
Josie Cohen, ActionAid's biofuels campaigner, said: "We've calculated that adverts on buses that pass the Department for Transport offices more than 2,000 times should be just about enough to ensure the staff can't look out of the window without our campaign hitting home."
She added that as many of the buses already run on the "destructive fuel", they are the perfect vehicles to deliver the message.
ActionAid is also urging members of the public to take part in a text campaign to discourage the government from increasing the UK's reliance on biofuels.