Of military response to counter Russia ‘challenge’

and Iran would be held responsible too. The Prime Minister also refused to rule out military action as Western allies discuss how to respond to the attack which killed up to 60 people and left more than 1,000 needing hospital treatment.

The alleged use of chemical weapons by the regime risks further escalating the crisis in the region.

When President Bashar al-Assad was last accused of launching a chemical weapons attack in April last year, Britain backed US President Donald Trump’s decision to bomb Syrian air force bases.

Yesterday, Mrs May branded the Douma attack “horrific” and “absolutely appalling”, and said international allies were working “urgently” on investigating the assault.

An emergency meeting of the United Nations Security Council was held yesterday.

Mrs May warned Russia, the main ally of the Syrian government, that it will face repercussions as well as President al-Assad.

She said: “If they are found to be responsible, the regime and its backers, including Russia, must be held to account.

“It is an absolutely appalling attack and if this is at the hands of the Assad regime, it is yet another example of the brutality of that regime and the way in which they have treated their own people.”

The PM said the attack in Douma was part of a “troubling wider pattern of acts of aggression” and abuse of international norms on the use of chemical weapons. Russia’s repeated use of its veto at the UN has “enabled” international rules on chemical weapons attacks to be broken and investigations hampered.

Mrs May said the poisoning of Sergei and Yulia Skripal showed “similar recklessness last month with the use of chemical weapons” in Salisbury.

“The UK’s case for holding Russia responsible for the attempted murder of Sergei and Yulia Skripal is clear,” she added.

“No other country has a combination of the capability, the intent and the motive to carry out such an act.”

Mrs May, who was in Copenhagen for talks with prime minister Lars Rasmussen, said they had agreed to do more to deal with the “growing challenge” from Russia to international security. Meanwhile, MPs have been increasing pressure to support military action.

Commons Foreign Affairs Committee chairman Tom Tugendhat warned: “Standing by as kids are gassed isn’t pacifism, it’s tolerating evil.”

However, Labour leader Jeremy Corbyn again refused to condemn Russia or Syria over the attack.

He said: “The United Nations has called for an urgent and rapid inquiry into it and indeed the reopening of inquiries into previous uses of chemical weapons. The tragedy and the terror of people’s lives in Syria can only end by a political solution.”