President Bashar al-Assad continues to retain hundreds of tonnes of his country's chemical stockpile after deceiving United Nations inspectors sent in to dismantle it, Syria’s former chemical weapons research chief told the Telegraph in an exclusive interview.

Brigadier-General Zaher al-Sakat – who served as head of chemical warfare in the powerful 5th Division of the military until he defected in 2013 – said that Assad’s regime failed to declare large amounts of sarin and its precursor chemicals.

Syria handed over what it said was its entire chemical arsenal to the UN’s Organisation for the Prohibition of Chemical Weapons (OPCW) in 2014 under a deal negotiated by the US and Russia after hundreds of people were killed in a sarin gas attack in the outskirts of the capital, Damascus.

The agreement averted US military strikes and the Obama administration declared one of the world’s biggest chemical weapons stockpiles “100 per cent eliminated”.

And Assad insisted once again this week that the regime was not in possession of any chemical weapons.