Journalists at Fox News were under orders to cast doubt on any on-air mention of climate change, a leaked email obtained by a media monitoring group revealed today.

According to the email, obtained by Media Matters, Fox News's Washington bureau chief, Bill Sammon, imposed an order to make time for climate sceptics within 15 minutes of the airing of a story about a scientific report showing that 2000-2009 was on track to be the hottest decade on record.

Media Matters said the bureau chief's response to the report exhibited a pattern of bias by Fox News in its coverage of climate change.

It also noted the timing of the directive. The email went out on 8 December last year, when the leaders of nearly 200 countries met in Copenhagen to try to reach a deal on climate change.

The email reads: "We should refrain from asserting that the planet has warmed (or cooled) in any given period without IMMEDIATELY pointing out that such theories are based upon data that critics have called into question."

It goes on to say: "It is not our place as journalists to assert such notions as facts, especially as this debate intensifies."

Media Matters, which earlier this week released an email from Sammon to staff on framing coverage of the health care debate, said the directive on climate change reporting exposed the network's bias.

In addition to the email, it said Fox had tried to delegitimise the work of climate scientists in its coverage of the hacked emails from the University of East Anglia. The network had displayed a pattern of trying to skew coverage in favour of the fringe minority which doubts the existence of climate change, Media Matters said.