Three senior British politicians are taking their campaign for Nasa hacker Gary McKinnon to the American ambassador in London.
Labour MP Michael Meacher, Conservative David Davis and Liberal Democrat Chris Huhne are planning to send a letter to US ambassador Louis Susman asking for an audience to discuss the case, a spokesman for Meacher said.
The cross-party group of politicians is pushing for the US to end its extradition proceedings against McKinnon on human-rights grounds.
"Michael Meacher, David Davis and Chris Huhne intend to seek meetings with the US ambassador," the spokesman told ZDNet UK on Thursday. "The letter will be sent in the next couple of days."
On Wednesday, the MPs met with home secretary Alan Johnson to urge him to intervene in the case and allow McKinnon to be tried in the UK. The Londoner has been accused by the US of causing $700,000 (£400,000) damage by hacking into military systems.
McKinnon, who has been diagnosed with Asperger's syndrome, denies the damage, and has said he only accessed the systems to look for evidence of UFOs.
In August, Johnson said he could not as home secretary step in to halt the extradition as this would be illegal given the serious nature of the accusations against McKinnon. But at the meeting on Wednesday, Johnson acknowledged he did have the power to stop proceedings, Meacher's spokesman said.
"[Johnson] did accept that it would be possible for him to intervene and that it wasn't unlawful for him to intervene, but claimed the limits of his discretion meant he had to be governed by law and precedents," said the spokesman. "He was concerned that precedents would be set for terrorists."
In a blog post on Wednesday, Meacher said Johnson felt his scope for intervention was narrowed by Article 3 of the Convention on Human Rights, which limits interference in extradition to cases where the subject is at real risk of execution, torture, or inhuman or degrading treatment.
The three politicians came away from the meeting feeling that Johnson had been prepared to listen to their case, and that it "wasn't the end of the road", Meacher's spokesman said.
On Thursday, the Home Office maintained that it would be unlawful for the home secretary to halt the proceedings, given a High Court decision to allow the extradition in July.
"Mr McKinnon is accused of serious crimes, and the US has a lawful right to seek his extradition, as we do when we wish to prosecute people who break our laws," said the Home Office in a statement.
The Home Office added that previous home secretary Jacqui Smith had sought and received assurances from the US that McKinnon's health and welfare needs would be met if he were imprisoned there.
McKinnon's legal team has applied to the High Court for leave to appeal its July decision to the Supreme Court. The judges' decision on this application is due to be handed down on 14 September.
If the application is unsuccessful, McKinnon's lawyers have said they will apply to appeal to the European Court of Human Rights.