They were targeted by Russian state operatives with the nerve agent, novichok, in March 2018, but survived the attempt on their lives. Moscow has denied any involvement.

Dawn Sturgess, 44, died after being exposed to the chemical weapon in a discarded perfume bottle, months on from the original poisoning. She was found unconscious on Saturday 30 June 2018, with police later confirming she had been exposed to novichok.

Ms Sturgess died the following week on 8 July, while her partner, Charlie Rowley, was discharged from Salisbury District Hospital on 20 July.

Mr Rowley has since said he recalled giving his partner a sealed bottle containing the deadly nerve agent, thinking it was perfume after finding a discarded and expensive-looking bottle.

Former police officer Nick Bailey, who was the first person to enter Mr Skripal's home after the attack, was also poisoned and survived.

The judge leading the inquiry said the risk of another attack on the Skripals "is not properly controllable" as appearing in person could lead to their identities and whereabouts being exposed.

Lord Hughes of Ombersley said proper security would not be possible if they attended the inquiry and warned they must not give evidence remotely as their location could be worked out.

He also ruled out playing recordings of the 2018 interviews with either or both of the Skripals as it would "enormously add to the risk that someone will identify them".

In his ruling this month, he said there was an "overwhelming risk" of a physical attack on the Skripals.

He went on: "There is every reason to be satisfied that an attack similar to that which appears to have taken place in 2018 remains a real risk... if either Sergei or Yulia can be identified and their current whereabouts discovered."

In June, a preliminary hearing for the Dawn Sturgess Inquiry at the Royal Courts of Justice heard that the Sturgess family wanted the Skripals to give oral evidence to address "unanswered questions".

The judge wrote that the Skripals have provided further statements addressing specific questions the family had raised and that transcripts of police interviews with the father and daughter had been disclosed.

Lord Hughes of Ombersley went on: "I am quite satisfied that the risk to both Skripals of physical attack clearly outweighs the advantage to the inquiry of their giving oral evidence.

"The disruption and intrusion into their private lives, whilst alone not sufficient to dictate the conclusion that they should not give evidence, adds to the balance a further reason why they should not."

Read more:
Miracle recovery: How the Skripals survived
Salisbury poisonings five years on

The public inquiry hearing is due to begin at The Guildhall in Salisbury on 14 October.

Two Russian GRU agents, identified by UK police as being behind the attack, and using the names Alexander Petrov and Ruslan Boshirov, were charged in their absence with attempted murder, despite claiming on Russian TV that they were on a sightseeing tour to Salisbury Cathedral.

A third GRU agent, using the name Sergey Fedotov, who appears to have been the unit's commander, was also charged in his absence with attempted murder.

All three are presumed to be in Russia, which has no extradition treaty with the UK.

In 2021, police issued a further appeal for information, with a renewed focus on the movements of the perfume bottle in which the novichok was stored, the location of which was unknown between the attack on the Skripals on 4 March 2018 and when Mr Rowley said he found it on 27 June 2018.