According to Ukrainian military data, Russian occupying forces have used dangerous chemical substances more than 9,000 times since the start of the war — including 6,540 incidents last year alone. This has largely involved the use of grenades filled with riot-control agents such as CS and CN gas.

Ukrainian and European officials say Russian units have also at times used chloropicrin, a choking agent first deployed during World War I. "The concern, voiced quietly in allied capitals, is that a prolonged or stalemated war in Ukraine could tempt the Kremlin to resort to more dangerous battlefield weapons.

"It is fair to assume that Russia’s chemical weapons programme is still extant. If Novichok were used on a wider scale, it could have a massive impact," said Hamish de Bretton-Gordon, a former British army officer.

Former Commander of the UK Joint Forces Command, General Sir Richard Barrons, argues that a shift to more lethal chemical weapons would be a disadvantageous move for Moscow, offering little military benefit while carrying high costs.

"You attract attention for a war crime, you run the risk of reciprocation, and there’s a double-edged sword — you may introduce your own forces to risk," he said, noting that factors such as wind direction could turn the weapon against those deploying it.

Putin may turn to chemical weapons if war hits a dead end, The Times

However, Barrons added that the temptation to use such weapons could arise "if you’re in a situation where the stakes are extraordinarily high, a situation where national survival is at stake."

Wind change was a common issue in the trenches of WWI. Chemical weapons are a two edged sword.