In a last ditch attempt to win their freedom the NatWest Three have petitioned the US Government to release crucial new evidence that lawyers claim will clear them of Enron-related fraud charges.

News of the evidence - described by lawyers as “critical to establishing innocence” - has emerged just one day before the anniversary of the three former NatWest bankers’ extradition to Texas.

They remain under house arrest in the US city pending their trial, which is due to begin on October 20.

The three - David Bermingham, Giles Darby and Gary Mulgrew - are each charged with seven counts of fraud and face 35 year prison sentences if convicted.

The evidence the three have asked the US Government to produce relates to a transaction in which their former employer NatWest and investment bank Credit Suisse sold their stakes in a Cayman-registered company to Enron.

The transaction, which took place in March 2000, is at the heart of the US prosecutors case against the NatWest Three. According to the prosecutors, the three bankers “devised and executed a scheme to defraud NatWest” by recommending the bank sell its 50 per cent stake in the Cayman company for $1 million.

The prosecutors further allege that the three bankers knew this stake was worth “far more” and that they were planning to fraudulently obtain the difference.

Lawyers for the three men insist that the US Government has documents that refute these charges but refuses to release them.

Specifically, the lawyers say in a formal petition filed this week that documents exist showing that Credit Suisse valued its identical 50 per cent share in the Cayman company at “negative $1 million” just before the sale.

The lawyers say that this “independent, contemporaneous valuation” is essential to proving that the recommendation that NatWest sell its stake for $1 million was “not part of a fraudulent scheme” but a “reasonable and appropriate” price.

Credit Suisse subsequently sold its 50 per cent stake for $10 million, a fact that prosecutors have used as evidence that the NatWest Three’s $1 million recommendation was a scam.

However, the NatWest Three’s lawyers claim there is further evidence that shows the $10 million payment was designed to “reward” CSFB for devising this and other Enron deals.

The documents are also said to show that Andrew Fastow, Enron’s former finance director, personally approved this “reward”. If that is the case, the NatWest Three’s lawyers maintain, then the three could not have “devised and executed” the fraud, as the prosecutors allege.

CSFB said the latest request for evidence was “a disingenuous attempt to shift blame away from the three individuals”.

“The evidence establishes that Andrew Fastow conspired with them to cheat the limited partners [owners] including CSFB. CSFB was a victim of their fraud,” it added.

The lawyers claim that they know the documents exist because they were submitted as evidence in separate civil litigation relating to Enron. They claim the US Government has resisted previous attempts to release the documents leading to this week’s formal petition.

Mark Spragg, the NatWest Three’s UK lawyer, said: "It appears this evidence will expose the fundamental flaws in the US prosecutor's case against the three and lead many to once again question why their extradition ever took place."

There is no official deadline by which judges must decide if the US Government should be compelled to hand over the documents but it is understood that lawyers expect to know the outcome of their appeal within three weeks.

The US Department of Justice, which is bringing the case, would say only that its “response will come in court”.

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