Bombs with chlorine have become the Islamic State (ISIL) militant group's "chemical weapons of choice," the newspaper reported.
Chlorine burns the lungs and can kill when inhaled in large enough quantities.
Some of the bombs used in the Iraqi city of Tikrit by the ISIL reportedly contained chlorine, which raised concerns that the militants could produce similar explosives once they return to the United Kingdom.
Some 20,000 foreign nationals were estimated to be fighting for Islamic State in Iraq and Syria at the beginning of 2015, with about 500 Britons among them.
Syrian government forces have also been accused of using chlorine during hostilities in the country, despite the fact that the country joined the Organization for the Prohibition of Chemical Weapons (OPCW) and agreed to destroy its chemical weapons arsenal in 2013.