Former environment minister Michael Meacher has urged prime minister Tony Blair to put pressure on US president George W Bush over climate change.
Mr Blair flies to the US ahead of July's G8 meeting
He said Mr Blair needed to call in the political goodwill Mr Bush owed him for the UK's support over Iraq.
He said the US could not rely on Britain's support if it "ignores their fundamental concerns".
Mr Blair flies to the US this week for talks with the US leader ahead of July's G8 meeting in Scotland.
"The US cannot count, despite its hegemony and its unique power, on the automatic support - even from close allies - if the US ignores their fundamental concerns," Mr Meacher told The World at One on BBC Radio 4.
"We have been a very strong, some of us think almost too subservient, an ally over recent years and it's time, I think, for the political goodwill which has been generated by our support to be called in."
Mr Bush has consistently questioned the need for quick action on climate change, and the US has not yet ratified the Kyoto Protocol aimed at cutting down greenhouse gas emissions.
Mr Meacher said he believed Mr Blair was genuine in his attempts to try and tackle climate change, and that he should appeal to the self-interests of US industry.
"It's a question of how one gets the world's biggest economic power, which is in the Bush administration uniquely connected to the oil industry, to take a different view.
"There is a clean development mechanism for a huge new sweep of investment in developing countries and don't American companies want to be part of that?"
Mr Blair will be in the US capital on Monday and Tuesday.
Other issues to be discussed will be Africa, Iraq and the Middle East peace process.