Sergei Skripal
Some 21 people being treated for exposure to substance.
Amber Rudd tells MPs attempted murder will be met with 'robust' response.
Investigators continue to probe the source of the nerve agent attack.
The five things you need to know about politics today.
'We know exactly what we are doing.'
Royal source says Duke of Cambridge's decision predates spy poisoning.
Scotland Yard says police officer also fighting for life.
An assassination attempt, a set-up, or a sign he was involved in espionage?
Mystery also surrounds the deaths of his wife and son.
The five things you need to know about politics today.