Theresa May has given Vladimir Putin until midnight on Tuesday to explain the use of a Russian-made nerve agent in the Salisbury attack or face retaliation for “a brazen attempt to murder innocent civilians on our soil”.

The Prime Minister told MPs that the Government "has concluded it is highly likely that Russia was responsible" for the attempted murder of the former spy Sergei Skripal and his daughter Yulia last weekend.

And she revealed that an illegal “weapons-grade” nerve agent known as Novichok was used in what she described as “an indiscriminate and reckless act against the United Kingdom”.

Novichok is one of the most deadly chemical weapons ever developed, with some variants being eight times more lethal than VX, the chemical used by North Korea to assassinate Kim Jong Nam, the half brother of Kim Jong Un.