THE wife of the Portland businessman killed in a mysterious helicopter crash today refused to comment on claims he had become a police informant days before he died.

It was reported that Stephen Curtis, an international lawyer who ran Group Menatep, the company that controls Russian oil giant Yukos, had contacted the National Criminal Intelligence Service (NCIS) in London to offer his services.

But he had only met his handler twice before his helicopter crashed near Bournemouth Airport while he was returning to Dorset on the night of March 3.

The claims were made on Channel 4 News, which also reported that the National Crime Squad was investigating the circumstances of the crash in which pilot Max Radford also died.

The Air Accidents Investigation Branch (AAIB) is said to be keeping an open mind and that the cause of the crash may not be known for months because of the ferocity of the explosion.

Dorset Police, meanwhile, said they had no plans to investigate further. It had already been revealed that Mr Curtis, 45, feared for his life in the weeks leading up to the fatal crash.

He received death threats and believed his phones were being bugged by business rivals. He is believed to have been in the process of selling his London penthouse because it was too well-known.

Mr Curtis's widow Sarah told the Echo from her home, Pennsylvania Castle on Portland, that she would not be commenting on any of the claims made in the programme.

Mrs Curtis refused to co-operate with the programme-makers and said that she would not be speaking out on any aspect of her husband's life or business dealings.

Channel 4 news also claim to have seen documents that show how crucial Mr Curtis was to the fortunes of Yukos.

In 1999 he is reported to have drawn up the structures for the oil company's oil trading business, giving it control of the companies while disassociating from them in such a way that would save it disclosing taxable sales to the Russian authorities.

Mr Curtis took charge of the huge financial empire controlled by Menatep when oil tycoon Mikhail Khodorkovsky, the former boss of Yukos, was jailed in Russia after being charges with fraud and tax evasion.

The oligarch turned to Mr Curtis because he was beyond the reach of Russian officials and knew everything about the company's complex structure.

No photograph of the secretive Mr Curtis has been seen until now. It was taken 20 years ago and he repeatedly refused to have his photograph taken at any function in recent years.

His business dealings were also highly sensitive.

He was born in 1958 in Sunderland and studied economics and law at Aberystwyth University.

He was employed as a clerk at Chester Crown Court before joining international law firm Fox & Gibbons, where he built up portfolio of wealthy Middle Eastern clients.

He set up his own practice, Curtis & Co in Park Lane in 1994 but left four years later to take up a strategic role in Menatep.

But the company has become mired in controversy and is reported to have been the target of a personal attack by Russian President Vladimir Putin who was opposed by the powerful Khodorkovsky. Its offices in Moscow were raided again last week.

Mr Curtis was buried in the grounds of Pennsylvania Castle following a funeral service attended by 350 people at All Saints Church in Easton.

Among those who attended was another Russian tycoon Boris Berezovsky who told Channel 4 that he was suspicious of the circumstances surrounding the death of Stephen Curtis.

But he added that he doubted the truth would ever be revealed.