Three British bankers accused by US prosecutors of a multi-million pound fraud linked to Enron flew out of Britain today, accompanied by US marshals, despite a major campaign to head off their extradition.

The so-called NatWest Three, David Bermingham, Gary Mulgrew and Giles Darby, arrived separately at Croydon police station in South London before heading to Gatwick airport for an Air Continental flight to Houston that took off at 9.38am.

Their lawyer, Mark Spragg, said: "It’s a very sad day. They have said goodbye to their families and they have no idea when they are going to come back. It’s all so unnecessary, that’s the real problem here. This could easily have been tried in the UK."

Speaking outside the police station, Mr Bermingham, 43, whose wife was by his side, said he had been "absolutely bowled over" by the support he had received.

He told reporters: "It is a very sad day because most of you are British and you got let down today by your own Government."

Referring to Neil Coulbeck, a witness in the case who was found dead in parkland in East London on Tuesday, he said: "We got screwed but there is a real tragedy. A man has died. I’ll be coming home one day."

After leaving the police station this morning, Mr Bermingham’s wife Emma, 39, said that she felt "sad, sad to be British".

A BBC radio reporter aboard the jet said that the three men were seated in economy class at the very back of the plane, each flanked by a US air marshal who was preventing them from speaking to journalists.

The men and their lawyers claim their are being extradited on a treaty between Britain and the US which is unfair and one-sided, because the US has yet to ratify it. Baroness Scotland, the Home Office minister, arrived in Washington last night, to try to persuade the US to endorse the 2003 treaty, which was introduced primarily to deal with terror suspects.

In an emergency debate in the House of Commons yesterday, opposition MPs insisted that it was easier to extradite people from the UK than from the US. However, the Prime Minister denies that the US is receiving preferential treatment, and claims that the men would in any case have been extradited under the old rules.

Mr Blair also said that a deal between Lord Goldsmith, the Attorney General, and the US Department of Justice would ensure they were granted conditional bail.

The men's solicitor said that US prosecutors were suggesting they may get bail - but only to live in the Houston area, leaving them unable to work and to earn enough money to pay their lawyers. If the men are not granted bail, they could spend up to two years in jail before their case comes to trial, said Mr Spragg. And if found guilty they could spend up to 23 years in jail, with no chance of parole.

However, the rows over their bail conditions and the treaty were overshadowed yesterday by the death of Mr Coulbeck, 53, a former colleague of the NatWest Three. Civil rights campaigners said they were troubled by reports that he was being hounded by the FBI.

His friends said he had been interviewed in the UK by the FBI a number of times and felt under an immense amount of pressure. A spokeswoman for the Royal Bank of Scotland, which now owns NatWest, said there was no evidence that Mr Coulbeck was involved in the sale of the transaction under investigation.

Today forensic police officers took fingerprints from Mr Coulbeck's bedroom window as they scoured the family home in Woodford Green. Mr Coulbeck's wife Susan and grown-up sons Ralph and Christopher remained inside the family home, which is being guarded by police officers.

Mr Bermingham, Mr Mulgrew and Mr Darby are accused of conspiring with executives of the collapsed US energy company Enron to cheat their former employers, Greenwich NatWest, out of £11 million. It is claimed that they advised their employer to sell off its stake in part of Enron at well below its market value, then bought the stock themselves and sold it at a profit.

Enron collapsed in 2001 after admitting inflating profits and hiding debts. US prosecutors issued warrants for their arrest in 2002.

The case has been backed by a campaign from opposition politicians, who attacked the treaty and the men's unfair treatment in yesterday's three-hour debate in the House of Commons. A vote to adjourn the House of Commons was a symbolic protest against the Government. And on Tuesday, peers voted in favour of suspending extradition agreements with the US.

The votes will not alter the men's case but they do show the strength of feeling against the agreement.

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