Defence experts still do not know the "precise source" of the Salisbury spy attack but are "completely confident" the nerve agent used was Novichok.

Experts at Porton Down, the UK defence laboratory, are still trying to establish how the agent was administered to ex-Russian spy Sergei Skripal and his daughter Yulia on March 4.

On Monday, chief executive Gary Aitkenhead said the deployment was "probably only within the capability of a state actor".

His comments came after Russia issued a failed request to the UK to visit Ms Skripal.

Moscow's foreign minister Sergei Lavrov labelled the refusal "outrageous", adding that Britain was "putting all decency aside" as the countries expel each other's diplomats.

Mr Skripal remains critically ill in hospital, while his daughter continues to improve.