George Galloway breached broadcasting impartiality rules when he used his radio show to cast doubt on Russian involvement in the poisoning of Yulia and Sergei Skripal in Salisbury last year, according to media regulator Ofcom.

The former MP used his weekly programme on TalkRadio to repeatedly criticise claims of Russian involvement in the incident. He mocked those who agreed with the UK government that the Kremlin was behind the novichok nerve agent attack on the former Russian security agent and his daughter.

Ofcom said it was minded to sanction TalkRadio, which is owned by Rupert Murdoch’s News UK, for broadcasting the discussion last March without giving enough time to people with opposing views.

Galloway said the media regulator had engaged in a “transparently politically motivated attempt at censorship” that was designed to stifle his “lone voice” on the airwaves.

On the show, broadcast 12 days after the poisoning last March, Galloway repeatedly poured scorn on the British government’s claims of Russian involvement in the attempted murder and pointed out that Salisbury was close to the UK’s chemical research laboratory at Porton Down.

Galloway suggested there was no reason for Russia to have carried out the attack: “I’m not saying the Russian intelligence services are above such a crime,” he told listeners. “They have committed many such crimes, just like every other intelligence agency in the world, including – and perhaps particularly – our own. Don’t get me started on intelligence services. The question is, why?”

Galloway then read 11 text messages from listeners, all broadly supportive of his views on the events in Salisbury, according to Ofcom.

At one point, he broke from a discussion with an employee of the leftwing website The Canary to read a tweet from a London taxi driver that said: “I haven’t found a passenger who believes this story all week.”

TalkRadio argued that “Galloway’s colourful critique would be regarded by listeners as a highly opinionated personal view attack” and therefore did not require a counterbalance. It said Galloway’s producer made an effort to find guests to appear on the show and make the case for Russian involvement in the Salisbury poisoning, but “all of them declined”.

The radio station said it had since talked to Galloway about the need to include contrary voices on his show and, if a guest could not be booked, a TalkRadio presenter should come on air to challenge Galloway and provide an alternative viewpoint.

The media regulator has already announced its intention to sanction the Kremlin-funded RT television station for breaking impartiality rules following the Salisbury poisoning. Two of the breaches related to programmes hosted by Galloway.