One of the "NatWest Three" bankers facing extradition to America has accused the Attorney General of striking a cynical deal to sell out innocent Britons.

David Bermingham levelled the charge in a four-page letter to Lord Goldsmith in which he pleaded for the case to be heard in British courts, rather than in Texas.

The banker said he could see only two reasons why Lord Goldsmith would have agreed to the extradition - either to protect the reputation of the United States as a trusted partner, or in return for "some other favour elsewhere" from Washington.

In his letter, seen by The Sunday Telegraph, he wrote: "If either is the case, then may God forgive you, because on behalf of my wife and our three young children, I surely cannot."

Mr Bermingham, Gary Mulgrew and Giles Darby are under investigation over an alleged ?11 million fraud linked to the ?40 billion collapse of the energy giant Enron.

They stand to be extradited by July 17, possibly as early as the middle of this week, under a 2003 law which makes it easier for Britons to be sent to America for trial than for US citizens to be extradited to this country.

Because they fought extradition, they may be considered "fugitives" and locked up for two years in Texan jails awaiting trial - although Tony Blair has pledged to do what he can to ensure that they are granted bail.

The bankers want the Crown Prosecution Service to take over the investigation and prosecute in Britain, which would prevent a US trial. But their attempt to force the Serious Fraud Office to investigate was thrown out last month by the European Court of Human Rights.

Yesterday Lord Goldsmith, backed by the Home Office, rejected a demand from the Conservatives for the SFO to reconsider its decision, and insisted that the bankers had been treated fairly.

The Attorney General said: "The main evidence was in the USA; the conspiracy took place there; the alleged fraud could not have occurred without the complicity of the Enron executives; the American case was advanced; and it was in the overall interests of justice for it to be dealt with by one court."

Baroness Scotland, a Home Office minister, called the extradition decision "the fairest and most appropriate thing for our country".

The political row over the case has centred on why Britain enshrined into domestic law the terms of a 2003 extradition treaty with the US which has never been ratified by Washington. As a result, America can secure the extradition of British citizens without presenting evidence in court, whereas if Britain were to seek to bring an American citizen for trial in this country, evidence would have to be presented to a US court.

Mr Bermingham told Lord Goldsmith that the arrangements gave US prosecutors "the power to destroy someone's life without checks or balances". And he wrote: "There can only be two reasons that you would not allow a prosecution in this country to go ahead. The first is that you know full well that we are innocent, but for political reasons cannot allow that to be proved, because it would cast into doubt all your ministers' assertions that the US is a trusted extradition partner and should be allowed to extradite without evidence.

"The second is that you have long ago agreed with the Americans that we should go to America, perhaps in return for some other favour elsewhere."

The bankers' lawyer, Mark Spragg, said Lord Goldsmith's decision would "send a shiver down the backs of anybody in corporate finance or the City". David Davis, the shadow Home Secretary, called the extradition arrangements "one-sided".