Russia suggested that poisoned former spy Sergei Skripal could be being detained by the British state unless its officials are allowed to speak to him following his discharge from hospital.
Mr Skripal, 66, and his daughter, Yulia Skripal, 33, were admitted to hospital after coming into contact with the military-grade nerve agent novichok.
The pair were left fighting for their lives after being found unconscious on a park bench in Salisbury, Wiltshire, on 4 March.
Yesterday morning, it was confirmed by the health authorities that Mr Skripal had left Salisbury District Hospital, following his daughter’s discharge on 10 April.
Theresa May has said it is “highly likely” that Moscow was behind the attack but Russia has denied involvement in the incident.
Russian president Vladimir Putin wished Mr Skripal “good health” but suggested he would have “died on the spot” if novichok had been used.
Moscow’s ambassador to the UK, Alexander Yakovenko, demanded the right to see the Skripals, claiming the UK was flouting international law by refusing consular access.
He highlighted the 1963 Vienna Convention, which gives consular officials access rights if one of their nationals is in prison, custody or detention. Mr Yakovenko said: “We are saying that they are isolated because we don’t have access to them.
“This is our interpretation. You can call it detained, you can call it isolated, you can call it kidnap.
“Unless we see them it is difficult to make a conclusion.”
He acknowledged that the Foreign Office had told him they did not interpret the situation in the same way and added: “I got the impression that we will never see them”.
Scotland Yard has said it will not discuss “any protective or security arrangements” put in place for the Skripals.
Detectives from the UK’s Counter Terrorism Policing network continue to investigate the attempted murder of the pair.
A Scotland Yard spokesman said: “This is a complex investigation and detectives continue to gather and piece together all the evidence to establish the full facts and circumstances behind this dreadful attack.”