Foreign secretary Jeremy Hunt will on Thursday declare the Russian government-owned TV station RT to be a “weapon of disinformation” in a speech to mark World Press Freedom Day.

The comments, to an audience in Ethiopia, mark an escalation of a British ministerial assault on the standards of the Russian broadcaster, originally known as Russia Today, which had faced repeated investigations into its output by the media regulator Ofcom.

“After the Russian state carried out a chemical attack in the British city of Salisbury last year, the Kremlin came up with over 40 separate narratives to explain that incident,” he is expected to say in Addis Ababa.

“Their weapons of disinformation tried to broadcast them to the world. The best defence against those who deliberately sow lies are independent, trusted news outlets.”

The foreign secretary said it remained a matter for Ofcom to independently decide whether the station should be closed down. At the end of last year RT was found guilty of seven breaches of the British broadcasting code in relation to programmes broadcast in the aftermath of the Salisbury novichok poisoning.

A spokesperson for the media regulator reaffirmed its independence and said it has yet to decide on a punishment, which could include fining the network or removing its licence to broadcast in the UK, although the latter option is considered highly unlikely.

Hunt’s comments could exacerbate the high-stakes dispute. Russia has retaliated by opening an investigation into the BBC’s right to operate its Russian service out of Moscow, while RT is seeking a judicial review of the Ofcom decision in the British courts.

The Conservative politician has made media freedom one of the signatures of his tenure as foreign secretary and is hosting a ministerial conference on the issue on London later this year, in addition to enlisting the help of human rights lawyer Amal Clooney to act as his special envoy on media freedom. Earlier this week Hunt was photographed with undercover Ghanian journalist Anas Aremeyaw, who does not show his face in public.

The foreign secretary will use his speech to accuse the Russian state of “manipulating the media, spreading fake news” and say thatRT is a key part of that effort.

“Channels like RT – better known as Russia Today - want their viewers to believe that truth is relative and the facts will always fit the Kremlin’s official narrative,” he will say.

“Russia in the last decade very disappointingly seemed to have embarked on a foreign policy where their principal aim is to sow confusion and division and destabilise fragile democracies.”

He is also expected to reference a baffling interview conducted by RT editor-in-chief Margarita Simonyan with the two purported Salisbury poisoning suspects, in which the Russians insisted they were visiting the Wiltshire city to look at the cathedral’s famous spire: “Hilarious though it was when we had the interview with the Salisbury suspects, there was actually a much darker purpose behind all of this.”

Explaining his broader thinking, Hunt will say media freedom is not a “western” value but instead a “force for progress from which everyone benefits”.

The UK recently rose to 33rd on Reporters Without Borders’ World Press Freedom index, although the report’s authors warned that the country remains one of the worst-ranked western European countries “largely due to a heavy-handed approach towards the press, often in the name of national security”.