The start of the inquest into the death of poisoned spy Alexander Litvinenko has been postponed until 2 October.
The inquest on the former KGB agent was due to begin in May, but coroner Sir Robert Owen said today that he had put it off for five months “with great reluctance”.
A hearing at London's Royal Courts of Justice was told that all the material needed for the inquest would not be ready in time for a May start.
Mr Litvinenko, 43, was poisoned with polonium-210 while drinking tea at the Millennium Hotel in London's Grosvenor Square.
His family believes he was working for MI6 at the time and was killed on the orders of the Kremlin.
At today's pre-inquest hearing, Sir Robert heard that delays over disclosure of evidence meant a start date of May was "unrealistic".
He looked at the possibility of hearing some parts of the inquest while evidence was still being compiled in others, but said this was not possible.
"I have therefore come, with great reluctance, to the conclusion that I can no longer adhere to May 1 as the starting date," he ruled.
"The realistic date on which I can be confident of making an effective start is the October 2 of this year."
He said there could be "no departure" from the date.
"I have the greatest sympathy for the position of Marina Litvinenko and her son Anatoly.
"I recognise that she has a right to a prompt resolution of this matter to understand the circumstances of the death of her husband.
"But as I know she understands, the thoroughness of this investigation must not be compromised."
Turning to Mrs Litvinenko, he said: "Mrs Litvinenko, I am acutely conscious that it will come as a great disappointment to you for this matter to be deferred for a further five months."
But he said he was "motivated solely by my determination to carry out a full, the fullest, investigation into the circumstances of your husband's death."
Speaking outside the Royal Courts of Justice, Mrs Litvinenko said despite the delays, she still had faith in the inquest process.
She said she was not surprised by the further delay, despite it being more than six years since her husband's death.
"It's not easy, but I do understand," she said.
"So many things happened already. It started in 2011, it did not happen then.
"I still have faith. I have an extraordinary team and I appreciate the job they do for me."
The court today also heard how former KGB agent Andrei Lugovoy has announced he will no longer help the inquest.
It has been claimed that the Russian politician and another former KGB agent Dmitry Kovtun poisoned Mr Litvinenko, but attempts to extradite Mr Lugovoy to the UK have been rejected by the Russians.
Mr Lugovoy announced in a press conference earlier this week that he will no longer assist the inquest, saying he had lost faith in the possibility of an "impartial investigation".
But Mrs Litvinenko today said she was not surprised by his decision.
Family friend Alex Goldfarb said: "People should not forget Mr Lugovoy is not just a regular politician.
"He is a man who has been accused of murder and his overwhelming interest here is the interest of someone who has to answer the charges."
He said as an "interested party" up until now, Mr Lugovoy had been able to familiarise himself with all of the documents being disclosed, including the police file against him.
During the hearing, the coroner said Mr Lugovoy appeared to be labouring under a misapprehension about various aspects of the inquest, including whether he could get "justice" or not, and whether parts would be heard in secret.
"He appears to assert that there will be court sessions held in secret," Sir Robert said. "That is a misunderstanding as to the nature of an inquest and the application that has been made on behalf of Her Majesty's Government."
The coroner has been asked to rule whether certain documents should not be disclosed because they pose a risk to national security.
In order to make a decision, he has said he will examine some of the evidence in private, before ruling whether it should be included and made public, or not included at all.
During today's hearing, counsel for the inquest Robin Tam QC said it was "unrealistic in the extreme" to expect it to start in May.
He said delays getting material from various Government departments had proved a "source of some frustration", and it was expected that it would all be collected by the end of July.
Material from the Metropolitan Police is expected to be collected and analysed by mid-July, he said.
"It's probably realistic to expect to be ready for a start date in the autumn," said Mr Tam.
Henrietta Hill, for Mrs Litvinenko, said the widow and her son had an "entirely human need" to understand what had happened.
"We find ourselves already in a position where it is over six years since he died," she said.
"We find ourselves in a position where it's almost six years since the Metropolitan Police completed their investigation."
She said there was a "grave disappointment" that various Government departments had appeared to ignore the coroner's requests for material - the court heard some had been asked for evidence last January, but had only started disclosing it in October.
Neil Sheldon, for the Home Office, told the coroner: "Lest there be any doubt about it, may I be permitted to assure you that Her Majesty's Government accords your investigation the highest priority.
"It is clearly appropriate and necessary that no conceivable stone is left unturned.
"We continue to work with your team to make sure they are turned."