America's most influential right-wing media outlet is facing up to the chilling prospect of having a whistleblower in its own newsroom.

Two internal memos from Fox News' Washington bureau chief, Bill Sammon, have found their way into the public domain in recent days via the liberal website Media Matters.

They detail how the news organisation which bills itself "fair and balanced" has made its reportage anything but.

At issue in the supposedly-secret directives is semantics; specifically the language that Fox correspondents use when discussing two of the most controversial items on Barack Obama's agenda: healthcare reform and climate change. Sammon, according to critics, at least, is shown by the emails to be ensuring that coverage of the Obama agenda carries a hostile bias. That would directly contradict Fox's official policy, which has always been to insist that while commentators such as Bill O'Reilly and Glenn Beck are dyed-in-the-wool conservatives, its news teams are unbiased.

The first memo, made public last week, was sent by Sammon last October, shortly after the US Senate unveiled a health bill which included a provision that would give the public sector a small role in providing medical insurance to Americans.

This provision was widely described at the time as the "public option", but Sammon decreed that Fox should instead describe it using one of three sanctioned phrases: "government-run health insurance", "government option" or "so-called public option".

All three of those phrases had previously been identified by Republican Party researchers as polling badly among the US electorate. The phrase Sammon outlawed ("public option") by contrast inspired positive reaction.

The second directive from Sammon leaked on Wednesday. It was sent to Fox staff in the run-up to this year's Copenhagen climate summit and ordered that any mention of climate change on the station should be coupled with an expression of scepticism regarding its very existence.

Reporters 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", the memo read. "It is not our place as journalists to assert such notions as facts, especially as this debate intensifies." All but a tiny handful of global warming sceptics accept that the earth's temperature has risen in recent decades.

Sceptics instead tend to deny that temperature change is linked to human activity and claim it is, in any case, rendered insignificant when viewed against longer-term trends.

So Fox duly found its journalistic practices in the firing line. "The network's coverage regularly gives unwarranted weight to anti-scientific claims regarding climate change," read a statement from the Union of Concerned Scientists. Al Gore, for his part, published a blog entry, claiming: "Fox News has consistently delivered false and misleading information to its viewers about the climate crisis. The leaked emails now suggest that this bias comes directly from the executives responsible for their news coverage."

Sammon's line on climate change contradicts that of Fox owner Rupert Murdoch. In 2007, Murdoch said: "Climate change poses clear, catastrophic threats. We certainly can't afford the risk of inaction."

The world according to Fox

On his radio show last week, Fox commentator Glenn Beck alleged that 10% of Muslims are terrorists, asking, "Why isn't this receiving coverage?" The reason: the real figure is closer to 0.1%.

Last November, Fox host Gregg Jarrett told viewers to marvel at "huge crowds" attending the launch of Sarah Palin's memoir Going Rogue. Accompanying his spiel was footage of crowds watching Palin speak. But that footage had been shot during the 2008 election. The network apologised, blaming a "production error".

A year ago, Republican lawmakers held a rally in Washington opposing Barack Obama's healthcare reform. Fox host Sean Hannity spoke wistfully of the vast turnout of "between 20,000 and 45,000 people". But eagle-eyed viewers noted that crowds depicted on screen were in fact attendees at an entirely different (and much larger) event which had taken place two months earlier. Hannity later apologised for "an inadvertent mistake".