A judge has given doctors permission to take blood samples from Sergei Skripal and his daughter Yulia so that tests can be carried out by chemical weapons experts, following a hearing in a specialist court.
Mr Justice Williams has made a ruling following a hearing in the Court of Protection, where issues relating to people who lack the mental capacity to make decisions are considered, in London.
He said both Mr and Ms Skripal were unconscious in hospital in Salisbury and therefore unable to give their consent to blood samples being taken or tested.
The judge, who is based in the Family Division of the High Court in London, announced his decision on Thursday after analysing the case at a private hearing earlier this week.
Mr Justice Williams said doctors at Salisbury NHS Foundation Trust could provide blood samples to the Organisation for the Prohibition of Chemical Weapons (OPCW).
He said trust bosses could also provide copies of medical notes to the OPCW.
The judge said he had concluded that such moves were in the best interests of Mr and Ms Skripal.
Mr Justice Williams said the UK Government had invited OPCW, which is based in The Hague, Holland, to send experts to assist with investigations.
He said OPCW experts wanted to analyse fresh blood samples from Mr and Ms Skripal.
The judge said bosses at Salisbury NHS Foundation Trust had told government officials that a Court of Protection judge would have to authorise such moves because Mr and Ms Skripal were incapacitated.
He said Government lawyers had then asked him to make decisions.