What is Novichok and how has it been used?

Police teams take swabs in Salisbury after former Russian spy Sergei Skripal and his daughter Yulia fell ill on a park bench in the city (Ben Birchall/PA)

Questions have been raised again about Novichok after German authorities said Russian opposition leader Alexei Navalny was poisoned with the nerve agent in England.

Mr Navalny, one of the most prominent critics of Russian President Vladimir Putin, fell ill on August 20 and was transferred to a Berlin hospital where tests found he had been poisoned.