Boris Johnson has said he fears Vladimir Putin’s “cynical, barbaric” regime may resort to deploying chemical weapons in Ukraine.

       As Western allies express concerns Russia could use the banned weapon, the prime minister stressed Moscow had a “fake story ready to go” — with claims of its opponents storing chemical weapons.

       After the UK government accused the Kremlin of war crimes following the bombing of a maternity ward in Ukraine’s port city of Mariupol, Mr Johnson said it showed Putin was prepared to reject “all norms of civilised behaviour”.

       He also revealed Dominic Raab, the deputy prime minister, would soon travel to Hague to see “what evidence” of war crimes Britain could help supply the International Criminal Court (ICC), which recently launched an investigation into Russia’s invasion of Ukraine.

       Speaking to Sky News presenter Beth Rigby, Mr Johnson said: “The stuff which you are hearing about chemical weapons is straight out of the Russian playbook.

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       “They start saying that there are chemical weapons that are being stored by their opponents or by the Americans so that when they themselves deploy chemical weapons, as I fear they may, they have a fake story ready to go.

       In a reference to the Salisbury nerve agent attack, the prime minister added: “You have seen it in Syria, you saw it even in the UK.

       “I just note that is what they are already doing. It is a cynical, barbaric government I’m afraid.”

       Mr Johnson appeared to be referring to comments from the Russian foreign ministry spokesperson Maria Zakharova.

       Earlier this week, she accused Ukraine, without evidence, of running chemical and biological weapons labs in its territory with the support of the US — a claim dismissed by the White House as “preposterous”.

       His remarks also come after Western officials first sounded the alarm on Wednesday over the potential use of chemical weapons, as they highlighted that the Russian military were “burning through a lot of high-end kit” amid a fierce Ukrainian resistance to the Kremlin’s invasion.

       And earlier, James Heappey, an armed forces ministers, hinted the use of chemical weapons may cross a “red line” which could trigger an international response.

       He said: “I don’t think it’s helpful to get into any firm commitment right now about where that red line sits, but I think President Putin needs to be very clear that when other countries have used chemical weapons it has caused an international response.”

       “I think he should reflect very urgently on what has happened to other countries where they have used them.”

       This map shows the extent of Russia’s invasion of Ukraine

       (Press Association Images)

       Mr Johnson also revealed to Sky News he had had some “very frank” and “deeply upsetting” conversations with the Ukrainian president, Volodymyr Zelensky, including over issues such as a no-fly zone, which has been ruled out by Western leaders over fears of a direct escalation with Russia.

       “It’s terrible. I gotta say it’s absolutely terrible. And I’ve got a massive admiration for Volodymir Zelensky,” the prime minister replied, when asked whether it played on his conscience.

       “I think any leader in his position would be saying exactly the same thing. They’d be saying, why can’t you provide that air cover? Why can’t you, as the West, simply help to clear our skies of Russian planes and stop us being bombarded from the air, stop this, this evil going on in Europe?”

       He added:“The difficulty is that there is a line beyond which, quite frankly, the the UK and NATO would be would be deemed to be in conflict, direct conflict, with Russia”.

       Asked whether the use of chemical weapons was a “red line” that may result in the government changing its approach on a no-fly zone, the prime minister replied: “It’s agonising. It’s absolutely agonising.

       “And I’ve had this conversation at least a couple of times now with with the Volodymir, but I think the difficulty is that it will require me to order RAF jets, UK pilots into the air with a mission to shoot down Russian fast jets.”

       Pressed again, he went on: “I think we’ve got to be realistic and we’re going to do everything we can to support the Ukrainian people, to support the amazing heroic Ukrainian resistance.

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       “And we’re doing more and more with support from - with air defences, with anti tank weaponry and all that kind of thing… I don’t want to go into the details, but I think I think our viewers know roughly what we’re doing, it has been ramped up. But that is effectively a defensive measure and… there’s a line that is very difficult to cross.”

       Mr Johnson also said he could not “see a way out” of the Ukrainian conflict, which the foreign secretary Liz Truss has suggested may go on for several years, without Russian troops withdrawing and the Kremlin realising its “catastrophic mistake”.