"It is extremely serious. This is a war crime," Prime Minister David Cameron told the BBC in an interview broadcast Friday.
The White House Thursday said U.S. intelligence agencies believe "with varying degrees of confidence" Syria used the nerve agent sarin on a "small scale" but did not elaborate further. However, the United State said the information it has doesn't represent proof of chemical weapons use.
Britain's Foreign Office said it, too, had "limited but persuasive information from various sources" of chemical weapons use in Syria, the BBC said.
Syria is believed to have stockpiles of chemical weapons. Concern within the international community has risen in recent months about the safety of the stockpiles. While there have been accusations, there hasn't been confirmation chemical weapons have been used during Syria's 2-year-old civil war.
The BBC said Britain reportedly obtained samples from inside Syria that have been tested by the Defense Science and Technology Laboratory.
"Material from inside Syria tested positive for sarin," a Foreign Office spokesman said.
Syria's government and rebels have accused each other of using chemical weapons.
Sarin is a colorless, highly toxic nerve agent that can cause convulsions, paralysis and death within minutes if it is inhaled, ingested or comes in contact with skin or eyes.
The United Nations estimates at least 70,000 people have been killed since the Syrian conflict began in March 2011.