British Prime Minister Theresa May summoned her senior ministers to a special cabinet meeting on Thursday to discuss joining the United States and France in possible military action against Syria after a suspected poison gas attack on civilians.
U.S. President Donald Trump warned Russia on Wednesday of imminent military action in Syria, declaring that missiles "will be coming" and lambasting Moscow for standing by Syrian President Bashar al-Assad.
Russia has warned the West against attacking its Syrian ally, which is also supported by Iran, and says there was no chemical attack in the Syrian town of Douma.
May recalled the ministers from their Easter holiday for a special cabinet meeting in Downing Street later on Thursday to discuss Britain's response to what she has cast as a barbaric attack that cannot go unchallenged.
The BBC said May was ready to give the go-ahead for Britain to take part in action led by the United States without seeking prior approval from parliament. Downing Street spokesmen repeatedly declined to comment on that report.
"The chemical weapons attack that took place on Saturday in Douma in Syria was a shocking and barbaric act," May told reporters on Wednesday. "All the indications are that the Syrian regime was responsible."
The Daily Telegraph newspaper said May had ordered British submarines to move within missile range of Syria in readiness for strikes against the Syrian military.
May is not obliged to win parliament's approval, but a non-binding constitutional convention to do so has been established since a 2003 vote on joining the U.S.-led invasion of Iraq. It has been observed in subsequent military deployments in Libya and Iraq.
Britain has been launching air strikes in Syria from its military base in Cyprus, but only against targets linked to the Islamic State militant group.
Parliament voted down British military action against Assad's government in 2013, in an embarrassment for May's predecessor, David Cameron. That then deterred the U.S. administration of Barack Obama from similar action.