A court hearing tomorrow, being described by lawyers as unprecedented in UK legal history, will decide whether the death of the man who took the last photograph of George Bernard Shaw must remain a mystery.

Allan Chappelow, 86, was found murdered in June 2006 in the home where he had lived in Hampstead, north London, for 72 years. Prosecutors have said, however, that they would rather let Wang Yam, a 45-year-old financier charged with Mr Chappelow's murder, walk free than risk having national secrets revealed at his trial, which is due to begin on Tuesday.

Last week an application to overturn the public interest immunity certificate – or gagging order – slapped on the trial by the Home Secretary, Jacqui Smith, failed. It means that much, or even all, of the trial will be held behind closed doors.

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Such is the stringency of the order that the media are even barred from revealing why it has been put in place, other than that it is for "national security" reasons.

The order has, however, raised serious concerns among lawyers and the media. Anthony Hudson, a barrister specialising in media law at Doughty Street Chambers in London, said the fundamental principle of a fair trial was that it is open to the public.

"One of the reasons is that the publicity surrounding a trial can bring forward more witnesses," he said. "Another is that witnesses are much more likely to be careful that their evidence is truthful if the trial is in public because there is a much greater risk a lie will be found out."

The gagging order was challenged by several media organisations, including The Independent on Sunday, and Mr Yam. Tomorrow lawyers acting for Mr Yam will go back to the Court of Appeal in a last-ditch attempt to overturn it.

Criminal lawyers say they cannot recall the Crown intervening in such a way in a murder trial before. Cases that involve breaches of the Official Secrets Act have been held in private for the self-evident reason that a public trial would expose the secrets. In a criminal trial, an application can be made to hold all or part of it behind closed doors on the grounds of national security, however.

"I'm not aware of a murder trial that has been held totally or partly in camera," Mr Hudson said. "It is certainly very rare, and very likely unique."

A reclusive man rarely seen by his neighbours, Mr Chappelow was dead for about a month before he was discovered among rotting furniture and piles of papers in his run-down £3m Georgian townhouse in June 2006. His head had been smashed in

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Police launched a murder inquiry, speculating that he was the victim of identity fraudsters.

They were alerted by his bank after thousands of pounds went missing, according to reports at the time. Nine days later a fire swept through the house. Mr Yam was arrested in Switzerland four months later and extradited to Britain, where he was charged with murder, burglary and deception, charges that he denies.

Whoever did murder Mr Chappelow ended the life of a lonely and vulnerable old man.

He had published several books, including two about the playwright George Bernard Shaw in the 1960s. He was also a freelance photographer, working for national newspapers in the 1950s. He took the last photograph of Shaw shortly before the playwright's death aged 94.

He inherited the house where he lived from the age of 14 from his antique dealer father and never married or had a girlfriend. He had little family.

A neighbour, Peter Tausig, said: "He always had gripes about his neighbours, the council or the post office, and would regularly fire off letters to them all."

His home was in a dilapidated condition – it was on the English Heritage At Risk register – because he did not like change. He had also been seen fixing broken tiles with adhesive tape.

After his death, another of his neighbours, Peggy Sparrow, said: "He was naive. He used to say to me how terribly clever I was to have had four children. To Alan, it was like a miracle. But he did occasionally have his eye on people. He once tried to get my sister to go on a holiday to Albania on his motorbike. She politely declined.

"In April we had a card from America. He said he had gone to see a long-lost cousin and was having a lovely time."