Former Home Secretary Priti Patel restricted disclosure of a “small set of documents” from an inquiry into the death of a British mother-of-three in a suspected Russian state-sponsored poisoning, it has emerged.

Ms Patel signed a restriction notice “without consultation” on July 27 in what the Dawn Sturgess Inquiry was told was an “exceptional measure”.

The lawyer acting on behalf of Ms Sturgess’s family said they are “whistling and dancing in the dark” and fear the inquiry into her death will not take place next year.

Michael Mansfield KC urged inquiry chair Lord Hughes, to “guard against” further delays in the case during a hearing on Friday.

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

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 restriction notice at the latest pre-inquiry review, held at the Royal Courts of Justice in London, Andrew O’Connor KC, counsel to the inquiry, said: “I would not wish to leave the subject of the restriction notice without observing that it is an exceptional measure.

“In most cases it ought to be you and not the Secretary of State who takes the decision – we would submit, a judicial decision – as to what evidence can be adduced in open proceedings, and what material must remain in closed.

“All that said, it should be emphasised that this particular restriction notice is a limited measure – it applies to only a very small proportion of His Majesty’s Government’s documentation that has been made available to the inquiry.”

Commenting on the delays in the inquiry so far, Mr O’Connor said the “difficulty and sheer length” of the disclosure process had been a “striking feature” of the case to date.

Ms Sturgess’s family questioned whether the inquiry had sufficient resources to cope with the thousands of documents that had been submitted for review.

In response, Mr O’Connor said: “Given the sensitivity of the material, there are limits to scaling up our capacity.”

He told the hearing they were making “very good progress” with police documents – adding that they had reviewed 23,000 documents to date.

Mr Mansfield urged the inquiry chair to be wary of a time “slippage” due to the consideration of sensitive material, saying: “People must have been thinking about what the sensitivities are, particularly in light of what is happening internationally at the moment.

“We are particularly whistling and dancing in the dark because we really don’t know what the exact problems are.

“This is not something that has suddenly appeared out of the blue and it has been a matter on a large number of people’s minds since the incident itself at the beginning.

“Last year, a large quantity of sensitive reports were being reviewed in order to decide whether to have an inquiry.

“There will have to have been a form of exercise for a decision to be taken to request an inquiry.

“Please can we guard against the (time) slippage.

“We don’t want to be saying we don’t understand the pressures but an approach different to the one that is being taken seems logical.”

Although a date has not yet been set, Lord Hughes ruled the substantive hearings would begin in Salisbury.

He said: “I’m quite satisfied these hearings should start in Salisbury and that the evidence of the family and of the immediate circumstances of this unfortunate lady’s death should be dealt with there.

“I’m equally satisfied that a large portion of the rest of the evidence ought to be taken in London.

“The reasons are essentially: one, security, and two, logistics.”

A further preliminary hearing is scheduled to take place at the end of March.