A witness in the case of the NatWest Three who has been found dead in an apparent suicide had been "hounded" by the FBI, it was claimed last night.

Neil Coulbeck, a former Royal Bank of Scotland (RBS) executive, had given evidence to the US law enforcement agency about the alleged fraud case.

He disappeared from his home in Woodford Green, east London, last Thursday and his body was found by a dog walker in nearby parkland on Tuesday.

One of the NatWest Three said he "hoped to God" the death was not linked with his case - but there were claims that Mr Coulbeck had been under pressure after repeated meetings with US agents.

The director of civil rights group Liberty, Shami Chakrabarti, said: "I am troubled by reports from Mr Coulbeck's friends that he was hounded by the FBI.

"I have been told that he had been interviewed more than once by the FBI and was under pressure partly for this reason.

"But I am greatly troubled by these reports and we are entitled to ask serious questions about the nature of FBI operations on British soil."

The dead man was found in parkland between a golf course and a wood near his home. He is thought to have taken his own life.

He has not yet been formally identified but wife Susan has been informed of the discovery by police.

One of the three men facing extradition today, David Bermingham, said: "I'm absolutely horrified. It is the most appalling tragedy.

"My heart goes out to his family. It is just awful beyond words."

Asked if he believed the death was linked with his legal case, he said: "I don't think I could live with myself if that was a factor."

He confirmed he knew Mr Coulbeck and added: "He was also a thoroughly nice, decent... one of the nicest guys you could ever wish to meet."

In a statement, RBS said: "We can confirm that Mr Coulbeck was employed by RBS until 2004, most recently as head of group treasury.

"There is no evidence that Neil was involved in the approval of the transaction under investigation.

"RBS has co-operated fully with all the appropriate authorities and made them fully aware of all the relevant facts in our possession.

"Neil was highly regarded by his colleagues here in RBS and was a respected, capable and hard working member of our senior management team.

"Our thoughts are with the Coulbeck family at this very difficult time.

In response to the allegations, an FBI spokesman said: "It is the policy of the FBI to not confirm or deny whether any individual was inter-viewed as part of an ongoing or pending investigation."

The NatWest Three are accused of an £11 million fraud in which their former employer NatWest was advised to sell part of a company owned by collapsed US giant Enron for less than it was worth.

Mr Coulbeck's wife was believed to be at the family home in Clematis Gardens, Woodford Green, yesterday afternoon being comforted by friends.

A neighbour in the wealthy suburb, 37-year-old Oonagh De Berry, said she was shocked by news of Mr Coulbeck's death.

The mature student said she knew Mr Coulbeck, 53, and his wife, and said they lived at the property with their two sons who were aged in their late teens or early 20s.

Ms De Berry said the dead man was "unassuming and shy", kept himself to himself and regularly jogged in parks.

"They seemed quite happy as a family. Only a few days ago they had a marquee in the garden and some kind of big party," she added.