For a television channel whose weekly reach is less than that of ITVBe +1, the profile of RT (formerly Russia Today) in the UK is conspicuously high.

The £310,000 it spent on an advertising campaign on London’s transport network – posters carried messages including “Watch RT and find out who we are planning to hack next” and “Beware! A ‘propaganda bullhorn’ is advertising here” – late last year no doubt helped.

One reason for RT’s high profile is its guests. Politicians from both the left and right appear on it or take its money. This is true for obscure, publicity-hungry backbenchers and prominent, on-message frontbenchers.

They do so, the argument goes, in order to deal with a media organisation that doesn’t show deference to “political and economic power”; to speak to constituents who shun “mainstream media”; or to show that they recognise the importance of “alternative” news and information sites.

Labour’s shadow chief secretary to the Treasury Peter Dowd was on the channel last week; the Tory MP and former chair of the Foreign Affairs Select Committee Crispin Blunt appeared on it in November. The former First Minister of Scotland Alex Salmond hosts a chat show; the Tory MP Daniel Kawczynski has called RT “one of my pet favourite channels”.

The Commons Register of Members’ Financial Interests shows that, in the past two years, MPs including Tory MP and chair of the Welsh Affairs Select Committee David TC Davies, Labour MP Rosie Duffield, and Tory MP and former deputy speaker Nigel Evans have been paid up to £750 an hour to appear.

Other politicians have appeared – including Labour’s shadow justice secretary Richard Burgon and shadow international trade secretary Barry Gardiner – without apparently being paid.

On Sunday, speaking on The Andrew Marr Show, the shadow chancellor John McDonnell said Labour MPs should stop appearing on RT as it “goes beyond objective journalism”. He is right. But quite why this only occurred to him over the weekend is unclear.

RT is a loosely-disguised arm of the Russian state and is its main propaganda outlet in the West. RT’s editorial policy rejects “objectivity”. There are “only approximations of the truth by as many different voices as possible” in the general media, according to Margarita Simonyan, the channel’s editor-in-chief.

Freed from the Manichean shackles of “truth” and “lies”, RT has been found in breach of the broadcasting code by Ofcom for repeated misleading and biased coverage of conflicts in Ukraine and Syria. In 2015, it alleged that BBC had staged a chemical weapons attack by President Assad’s forces in Syria.

Tom Watson, Labour’s deputy leader, has questioned whether the “combination of false or inaccurate stories and the alignment of editorial policy to that of President Putin’s Russian state” means that Britain needs a tougher regulatory regime to deal with RT.

But how does one regulate against a television channel for lying when lying appears to be its raison d'être?

McDonnell’s newly-voiced views have long been shared by some across Parliament. Boris Johnson, the Foreign Secretary, has called on MPs not to “validate and legitimate” RT, while Michael Gove, the Environment Secretary, has queried why frontbenchers are appearing on a “literal Kremlin propaganda channel”.

But the moral obligation to ostracise Russia’s propaganda is not shared by all in Parliament. Nor by Parliament itself; the channel is currently screened in Westminster.

In her statement in the House of Commons yesterday about the poisoning of Sergei and Yulia Skripal, the Prime Minister outlined a “well-established pattern of Russian state aggression” toward the West.

She accused the Kremlin of having engaged in repeated acts of military sabre-rattling and subversion, meddled in elections, hacked governments, and mounted a sustained campaign of cyber-espionage.

Russia also, she acknowledged, practises propaganda – or information warfare. Calls from MPs to boycott RT – or stop it from broadcasting in the country – featured heavily in the discussion that followed.

I know of a number of MPs who have appeared on RT entirely unaware of its close connections to Vladimir Putin’s regime. If RT’s true nature were not obvious prior to this week, however, then it certainly will be from how on. Ignorance is no longer an acceptable excuse, if indeed it ever was, for appearing on the channel.

But it does explain why RT – with its glossy-lipped presenters, and the same cutting-edge graphics and polished studios as other 24-hour news channels – has never been as toxic in Westminster as it should have been.

Perhaps this will change after the poisoning of the Skripals.

Dr Andrew Foxall is Director of the Russia and Eurasia Studies Centre at The Henry Jackson Society, a London-based international affairs think-tank