Defence minister James Heappey has warned Russia that the use of chemical weapons has "triggered an international response" in the past.
Western officials are concerned that Vladimir Putin could mount such an attack as the Russian tactics become increasingly brutal.
Mr Heappey told BBC Radio 4's Today programme: "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."
He added: "President Putin needs to be clear that the use of chemical weapons is just the most despicable thing that anybody can imagine.
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"As horrid as the pictures we are seeing on our TV screens today of an artillery strike against a hospital (are), they are but nothing by comparison to the suffering and devastation that chemical weapons cause."
A Western official, who spoke on condition of anonymity, said: “We’re seeing some of the disinformation come out of Moscow about non-conventional weapons.
“We saw this in Syria, the Russians starting that talk when they or their proxies were about to use those weapons there.”
Asked if there are concerns the Russia will use chemical weapons, they replied: “We have good reason to be concerned about the possible use of non conventional weapons.
“Partly because we’ve seen what has happened in other theatres, for example in Syria, partly because we see a bit of setting the scene for that in the false flag claims that are coming out, and some other indications as well.
“So it’s a serious concern for us.”
Russia-backed Syria was repeatedly accused of using chemical weapons in the country's bloody war, but Russia vetoed a 2017 UN resolution that would have looked into who was responsible.
It comes as talks were set to take place between Moscow and Kyiv foreign ministers in Turkey in what will be the highest-level discussions since the assault started last month.
