LONDON - Britain pledged yesterday to reform a peculiar legal power that lets judges order the arrest of visiting politicians and generals - a threat now focused on Israeli visitors and that, one day, might be invoked against Barack Obama or Vladimir Putin.

Lawyers working with Palestinian activists in recent years have sought the arrest of senior Israeli civilian and military figures under terms of "universal jurisdiction." This ill-defined legal concept empowers judges to issue arrest warrants for visiting officials accused of war crimes in a foreign conflict.

Their latest target is Tzipi Livni, Israel's former foreign minister and current opposition leader, who staunchly defends Israel's invasion of the Gaza Strip. Israel's government confirmed yesterday that Livni canceled a planned London trip this month after her office received news of a secretly issued arrest warrant awaiting her arrival.

British Foreign Secretary David Miliband later announced that Britain would no longer tolerate legal harassment of Israeli officials in this fashion.

Speaking after meeting Israel's London ambassador last night, Miliband said the British law permitting judges to issue arrest warrants against foreign dignitaries "without any prior knowledge or advice by a prosecutor" must be reviewed and reformed.

Miliband said the British government was determined that arrest threats against visitors of Livni's stature would not happen again.

"Israel is a strategic partner and a close friend of the United Kingdom," Miliband said. "We are determined to protect and develop these ties. Israeli leaders - like leaders from other countries - must be able to visit and have a proper dialogue with the British government."

Previously, British judges in private have accepted petitions from anti-Israeli activists to arrest Israeli Defense Minister Ehud Barak and Israeli army generals when they set foot in England. Barak successfully argued that he enjoyed diplomatic immunity as a serving government leader, while several other Israelis made U-turns for home when told of the warrants.

Legal experts in England and Israel say that "universal jurisdiction" could be abused endlessly to harass, if not effectively incarcerate, any high-profile visitor who oversaw a military or antiterrorist operation.

Experts said the legal concept would face decisive scrutiny if, instead of targeting Israelis, an arrest warrant targeted a past or current U.S. or European head of state.

Eugene Rogan, director of the Middle East Center at Oxford University in England, said the demands of global diplomacy required leaders to be able to travel abroad without facing arrest threats.

He noted that American officials had refrained from taking part in International Criminal Court proceedings in the Hague, Netherlands, out of fear that they might face arrest threats themselves.

Spain and Britain jointly pioneered the concept of universal jurisdiction when, in 1998, Britain executed a Spanish arrest warrant for former Chilean dictator Augusto Pinochet. British authorities kept Pinochet under house arrest in London until releasing him on humanitarian grounds in 2000.

Pro-Palestinian activists in Britain have long hoped to capture Israeli officials in the same net that once held Pinochet. "We cannot talk tough on terrorism and be weak on war crimes," said Chris Doyle, director of a lobbying group called the Council for Arab-British Understanding.

Livni, a onetime lead negotiator in Israeli diplomacy with Palestinian leaders, enjoys a dovish reputation in much of the West despite her support for invading Gaza to stop Hamas extremists' rocket fire.

She said in a speech yesterday, "I would make the same decisions all over again."