Postlethwaite appeared as a taxi driver in one of the Labour Party's political broadcasts during the 1997 general election. He marched in London against the Iraq War in 2003. He was an activist calling for action to prevent climate change. At the UK premiere of "The Age of Stupid" on 16 November 2009, he told then-Secretary of State for Energy and Climate Change Ed Miliband that he would return his OBE and vote for any party other than Labour if the Kingsnorth coal-fired power station was given the go-ahead by the government.