British Prime Minister Theresa May said on Saturday that it was right and legal to order cruise missile strikes against Syria after a chemical attack on civilians in the town of Douma.

May said the aim was to deter the Syrian authorities from further use of chemical weapons and to send a message to the wider world that it was unacceptable to use such weapons.

"It was both right and legal to take military action together with our closest allies to alleviate further humanitarian suffering by degrading the Syrian regime's chemical weapons capability," May said.

She added that she would address parliament on Monday.

May said intelligence and open source accounts indicated that the Syrian government was behind the attack in Douma last Saturday.

"Reliable intelligence indicates that Syrian military officials coordinated what appears to be the use of chlorine in Douma on the 7th April. No other group could have carried this attack," May said. "The [Syrian] opposition does not operate helicopters or use barrel bombs."