The chairman of the Russian-state Salisbury poisonings inquiry has said it is inevitable there will be “perhaps quite a lot” of material kept secret from the public.

Lord Hughes told a preliminary hearing for the Dawn Sturgess Inquiry that, although the starting point will be for everything to be heard in an open hearing, issues of national security and police workings will have to be heard in private.

He described it as “immensely frustrating” for those who can only attend public hearings.

Ms Sturgess, 44, died after being exposed to the nerve agent Novichok, which had been left in a discarded perfume bottle in Amesbury, Wiltshire, in July 2018.

It followed the attempted murders of former Russian spy Sergei Skripal, his daughter Yulia and ex-police officer Nick Bailey, who were poisoned in nearby Salisbury in March that year.

All three survived, as did Ms Sturgess’s boyfriend, Charlie Rowley.

The Metropolitan Police identified three suspects wanted in connection with the poisonings: Denis Sergeev, Alexander Mishkin and Anatoliy Chepiga, who used the aliases Sergey Fedotov, Alexander Petrov and Ruslan Boshirov respectively while in the UK.

Addressing the issue of the disclosure of documents, Lord Hughes told the preliminary hearing on Wednesday: “The starting point is it is an open hearing.

“But given what is alleged to have happened, there’s inevitably going to be some material, perhaps quite a lot, that falls into the closed category.”

Acting on behalf of Ms Sturgess’s family, Adam Straw KC argued that the more information put in the public domain, “the greater public confidence will be”.

Mr Straw added: “We readily accept the threat posed by Russia and the considerable amount of information that can’t be disclosed because of that.”

The family’s barrister also urged Lord Hughes to consider ordering the Government to identify Mr Skripal as a UK agent, despite its policy to “neither confirm nor deny”.

Mr Straw said: “It is an exceptional case… it is apparently completely clear to Russia that Sergei Skripal was a UK agent, not least because he was tried and convicted of being a UK agent.”

Another barrister acting on behalf of the family, Michael Mansfield KC, said the situation is like “Alice in Wonderland” – adding that it is “standing there denying the obvious”.

Cathryn McGahey KC, acting on behalf of the Government, said not applying “neither confirm nor deny” could put agents at “risk of death”.

“The more exceptions that are created, the weaker the protection of NCND,” she said.

Following the open hearing on Wednesday, parties were set to discuss the disclosure of documents in further detail behind closed doors.

Substantive inquiry hearings are due to begin in Salisbury in October next year.