Three former NatWest bankers have arrived in the United States to face fraud charges arising from the collapse of the energy giant Enron.
Giles Darby, David Bermingham and Gary Mulgrew - all expected to attend a bail hearing on Friday - deny wrongdoing.
They were handed over to US marshals at London's Gatwick Airport and arrived in Houston, Texas, around 2000BST.
The extradition of the NatWest Three has sparked a political row in the UK over US powers to extradite.
BBC correspondent Jon Brain, speaking by telephone from aboard the plane, said the men had been seated at the back of the aircraft - before other passengers had been allowed to board - and were surrounded by air marshals.
Critics have attacked the extradition system between the UK and the US as one-sided because the Americans have yet to ratify a treaty between the two countries.
Critics also say it is unfair the US authorities need only to outline the alleged offence, and provide "evidence or information that would justify the issue of a warrant for arrest in the UK" whereas British police must provide the American courts with evidence of "probable cause" if it wishes to extradite someone.
An emergency debate was held in the Commons to discuss the issue on Wednesday.
Prime Minister Tony Blair told the House of Commons that US prosecutors would not oppose bail if appropriate conditions were met.
But the men's UK solicitor, Mark Spragg, said it looked unlikely the men would get "meaningful bail".
"The US prosecutors are currently suggesting they may get bail but only to live in the immediate Houston area.
"That won't be satisfactory at all because they're going have to put up all their money and property as collateral even to get that sort of bail.
"They won't be able to work in the US so they won't be able to earn money to pay their lawyers so I fail to see how they can get a fair fight on that basis."
BBC correspondent Nicola Pearson said that, if the men were not granted bail, they could spend up to two years in jail before their case came to trial.
If found guilty, they could spend up to 23 years in jail without the chance of parole, she added.
Mr Darby, from Lower Wraxall in Somerset, Mr Bermingham, from Goring in Oxfordshire, and Mr Mulgrew, from Brighton in Sussex, are accused of making over £1m each out of a complex fraud in which their former employer NatWest was an alleged victim.
The body of another banker, who had been questioned by the FBI as a potential witness in the Enron fraud case, was found in London on Wednesday. There is no indication that his death is linked to the case.
Police said they were treating the death of Neil Coulbeck, who worked for the Royal Bank of Scotland (RBS) until 2004, as "unexplained".
The fraud case centres on a NatWest transaction under which it sold off part of its Enron unit. NatWest is a subsidiary of RBS.
RBS said: "There is no evidence that Mr Coulbeck 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."
The FBI said it would not comment while the case was ongoing.