The trio of British bankers known as the NatWest Three were jailed for 37 months each yesterday for their part in a complex multimillion-dollar fraud that made them a temporary cause célèbre.

Gary Mulgrew, David Bermingham and Giles Darby must also return the $7.3 million (£3.7 million) that they collectively made from the fraud, as part of a plea bargain agreed with prosecutors in November, that was formally approved by a Houston District Judge yesterday.

The three men admitted advising their former employer, NatWest, to sell part of the company controlled by Enron for a fraction of what it was worth.

NatWest was unaware that they had a personal interest in the buyer of the stake, which sold it on for a huge profit, and collectively netted them $7.3 million.

Although the NatWest Three only pleaded guilty to one of the seven counts of wire fraud against them, it represented a dramatic turnaround since they had fiercely maintained their innocence starting from September 2002.

Yesterday, they appeared to accept further responsibility for their actions by expressing a stronger sense of regret than in previous public statements.

“I clearly realise now that our involvement was wrong and we failed to take the right course of action,” Mr Darby told US District Judge Ewing Werlein at the sentencing hearing in Houston’s federal courthouse yesterday.

Mr Mulgrew added: “I believe I exercised a lack of integrity in my decisions. I’d like to apologise to the people who’ve been hurt by my decisions – my family, my friends and my ex-colleagues.”

Yesterday’s approval of the plea bargain was the latest development in a case that has dragged on for five and a half years.

In 2004 the trio became a cause célèbre as the most prominent target of a new extradition treaty between Britain and the US, which allowed America to extradite criminal suspects from the UK without prima facie evidence. The treaty was signed by David Blunkett in 2003 and came into effect in 2004. The treaty, and the ruling in September 2004 that the trio’s extradition should go ahead, was greeted by protests from British politicians, business people and human rights campaigners – in large part because it was not a reciprocal arrangement with the US. However, the US has since ratified the treaty at its end too.

The NatWest Three were extradited to the US in July 2006 to face charges that they plotted to deceive NatWest about the true value of the bank’s stake in the Enron partnership.

The trio will remain on bail until the US Federal Bureau of Prisons determines at which facility they will serve their sentences, a process that is expected to take about three weeks.

They will then begin their sentences immediately, at which point they will be permitted to apply formally to serve the latter part of their prison time in the UK.

The case against them was built in the US because it arose out of the myriad investigations into the circumstances surrounding the fraudulent collapse of Enron, the energy trader, in December 2001.

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