Theresa May was under pressure from ministers and allies to join a US-led military strike against the Assad regime as France set the pace for retaliation against a suspected chemical attack.

The prime minister said that President Assad and Russia would be “held to account . . . if they are found to be responsible” for the “barbaric” attack in Douma, Syria, in which at least 40 people including children died.

As Britain prepared options including the use of Tomahawk missiles, senior figures warned that it risked losing influence in Washington to France if it turned down a request by President Trump to join a retaliatory strike. Yesterday Mr Trump promised to respond “forcefully” to the use of a chemical thought to