Gary McKinnon arrives at Bow Street Magistrates court on May 10, 2006 in London. Theresa May, Britain’s top authority on domestic affairs, issued an unprecedented ruling blocking McKinnon’s extradition on humanitarian grounds Tuesday. (Bruno Vincent/GETTY IMAGES)

The British government took steps Tuesday to roll back a controversial extradition treaty with the United States, blocking the handover of an autistic computer whiz who had hacked U.S. military databases and moving to enact legislation that could complicate future attempts by American prosecutors to pursue alleged criminals in Britain.

The moves found the Conservative-led British government responding to critics who have long argued against what they call a “one-sided” 2003 treaty designed to make it easier for the United States to pursue terrorism suspects and other offenders on British soil. The proposed legislative changes could allow British courts to determine whether suspects should instead face trial here, effectively blocking extradition in some cases.

In a statement, the U.S. Justice Department said it was “disappointed” in the decision to block the extradition of the hacker, Gary McKinnon. But it sidestepped a direct response to Britain’s intention to limit the scope of the treaty, saying, “Our extradition relationship with the United Kingdom remains strong, as is demonstrated by the extradition of five alleged terrorists from the United Kingdom just last week.”

Theresa May, Britain’s top authority on domestic affairs, also outlined a sweeping move, first announced Monday, under which London would withdraw from a series of European Union justice and policing measures.

Tuesday’s developments pivoted around the decade-old case of McKinnon, 46, who is described by U.S. authorities as having committed “the biggest military computer hack of all time.” May issued an unprecedented ruling blocking the hacker’s extradition on humanitarian grounds. McKinnon, who has Asperger’s syndrome, was deemed by two psychiatrists brought in by May as being at high risk of committing suicide if sent to the United States, where he faces up to 60 years in jail.

May told the House of Commons on Tuesday that McKinnon was accused of “serious crimes,” but she said that he also was “seriously ill” and that the sole issue at stake was whether the extradition would “breach his human rights.”

“A decision to extradite would be incompatible with Mr. McKinnon’s human rights,” she said.

But May also proposed a wider shake-up to the extradition treaty with the United States, amid widespread concerns here that it is too easy for British citizens to be sent to the United States for trial and that a document that critics say was initially intended to cover only terrorism suspects is being used far more broadly by U.S. prosecutors.

Last year, an independent review led by retired judge Scott Baker concluded that the U.S.-British treaty did not operate in an “unbalanced” manner. But legal analysts here said sentiment that the treaty was lopsided persists, partly because U.S. prosecutors are more aggressive than their European counterparts in pursuing suspects outside their territory.

The measures outlined by May could take some time to come into effect, because proposed new legislation requires the approval of both houses of Parliament.

The Justice Department’s statement lauded U.S.-British cooperation on extraditions and appeared aimed at lowering any potential tensions with Britain over the matter.

To be sure, though U.S. officials were unlikely to be pleased by the decision, observers said the move to leave the treaty otherwise largely intact could assuage concerns at the State and Justice departments.

The rule changes, however, would not be retroactive and, therefore, would not have affected McKinnon’s case — hence May’s move to unilaterally block his extradition.

McKinnon has admitted to hacking into 97 U.S. government computers between February 2001 and March 2002, including those at NASA and the Pentagon. McKinnon, who has fought a 10-year-long legal battle, admits that he exploited security problems in U.S. government computers from his bedroom in a North London apartment but says he was looking only at files that would prove the existence of UFOs.

“He’s a classic computer nerd. He was looking for UFOs,” McKinnon’s mother, Janis Sharp, told the BBC on Tuesday morning. She later publicly thanked May for blocking her son’s extradition, saying: “Thank you, Theresa May, from the bottom of my heart. I always knew you had the strength and courage to do the right thing.”

May said British prosecutors would decide whether McKinnon would face trial in Britain.

But May also offered details about the announcement Monday of a dramatic overhaul to Britain’s policy on international extradition requests, saying Britain would opt out of more than 100 criminal justice measures with the European Union and then rejoin selected ones. The move appeared aimed at satisfying Conservative lawmakers who have grown increasingly skeptical of the E.U.’s reach in British affairs.

Yvette Cooper, who leads domestic affairs for the opposition Labor Party, said Monday that May’s decision put “internal party management” ahead of “crime fighting and the interests of victims.”

“Theresa May needs to explain why she wants to ditch the European Arrest Warrant, making it harder to stop criminals who flee abroad and harder to send home foreign criminals who have fled here,” Cooper said in Parliament. “And she also needs to set out clearly what she will guarantee in its place and whether she will just end up opting back in again in future.”