A Londoner accused of the "biggest military computer hack of all time" will tell Britain's highest court next week that United States prosecutors demanding his extradition have abused court processes in Britain. Gary McKinnon, 42, will ask the Law Lords to block his extradition on the ground prosecutors had unlawfully threatened him with a heavier sentence unless he agreed to surrender himself for trial in the US.

If his appeal is successful, the ruling may have implications for other British citizens, including business executives, wanted by American prosecutors. At the very least, it would stop prosecutors seeking to ratchet up sentences for defendants who resist extradition.

McKinnon is accused of hacking into 97 US military computers - including 16 NASA computers and one belonging to the Pentagon - from his bedroom in Wood Green in 2001 and early 2002. He is alleged to have crashed the US Army's entire Washington network of 2000 computers for 24 hours, significantly disrupting government functions.

Prosecutors also accuse him of shutting down 300 computers at a US Navy weapons station immediately following the terror attacks of 11 September, 2001. In subsequent media interviews, McKinnon said he had been looking for evidence of unidentified flying objects. He is not facing charges in Britain.

McKinnon's solicitor, Karen Todner, told the High Court last year of meetings at the US Embassy in Grosvenor Square. The legal attache, Ed Gibson, is said to have told her "off the record" that the New Jersey authorities were determined to see McKinnon "fry".

If her client pleaded guilty to two counts, he could expect a sentence of less than four years. Prosecutors would not oppose this being served in England, which would have meant his release halfway through. If not, she was allegedly told, "all bets were off" and McKinnon could expect a sentence of eight to 10 years "or possibly longer". There would be no chance of repatriation and, at best, only 15% remission.

McKinnon turned down the plea bargain. His QC, Edmund Lawson, referred the High Court last year to a Canadian case in which a US prosecutor had said a defendant who resisted extradition "was going to be the boyfriend of a very bad man" - understood to be a threat of homosexual rape in prison.

Lord Justice Maurice Kay and Mr Justice Goldring said they viewed the alleged approach to plea-bargain negotiations in this case "with a degree of distaste". From an English perspective, "the notion that a prosecutor may seek to induce a plea of guilty on the basis that substantial benefits will be withdrawn if one is not forthcoming is anathema".

McKinnon, who is on legal aid, will be represented before the House of Lords next week by leading human-rights lawyer David Pannick QC. He will tell a panel headed by Lord Phillips, the incoming senior Law Lord, that the proposed deal was more than just distasteful: it was an abuse of process. Extradition should not go ahead, Pannick will argue, if the requesting state had put improper pressure on a defendant to waive his right to resist extradition and plead guilty.

The US authorities will point out that federal prosecutors have a limited role in prisoners' requests for repatriation, and would not oppose one simply because a defendant had not consented to extradition. But McKinnon's lawyers will argue that a prosecutor who exaggerates his influence is just as guilty of an abuse of process as one who tells the truth.

It will be some weeks before their lordships deliver a ruling. But the fact they gave McKinnon's lawyers permission to argue abuse of process suggests his chances of winning his appeal are greater than his chances of being whisked to freedom in a flying saucer.