Brown said that Britain's principle of universal jurisdiction — a wide-ranging legal concept that allows judges to issue warrants for nearly any visitor accused of committing war crimes anywhere in the world — was being abused.
While heads of state and senior ministers enjoy immunity, pro-Palestinian groups have used the law to try to arrest former or retired Israeli officials, including Livni, who now serves as opposition leader, and retired general Doron Almog, who narrowly dodged arrest at Britain's Heathrow Airport in 2005.
Brown said the law was being abused by groups "who set out only to grab headlines knowing their case has no realistic chance of a successful prosecution."
Writing in The Daily Telegraph Thursday, Brown said "Britain cannot afford to have its standing in the world compromised for the sake of tolerating such gestures."
Israelis were outraged when an arrest warrant was issued for Livni in December ahead of her planned visit to the UK The warrant was revoked after she canceled her trip, but Israeli officials said the opposition leader's treatment — and British arrest threats against other officials and military commanders — risked poisoning relations between the two countries.
The Jewish state demanded that Britain change the law, which Israeli politicians described as absurd.
Britain has previously invoked universal jurisdiction to arrest and prosecute Afghan warlord Faryadi Sarwar Zardad, who was convicted and jailed for 20 years on charges of torture and hostage taking.
But Foreign Secretary David Miliband said last month that the rules could be used to threaten Britain's ability to hold face-to-face talks with leaders from other nations and promised to tweak the law.
Brown made no direct references to Israel in his editorial, but he noted that "there is already growing reason to believe that some people are not prepared to travel to this country for fear that such a private arrest warrant — motivated purely by political gesture — might be sought against them."
Brown said he would propose a change in the law which would restrict the right to prosecute war crimes covered under the universal jurisdiction laws to Britain's Crown Prosecution Service — effectively barring private groups from taking foreign VIPs to court. Brown said he would also toughen the standard of evidence required in such cases.
Diplomats here said Israeli officials have raised the issue with the UK, but insist that Brown's government already planned to address inconsistencies in British law which allow individuals to seek the arrest of others on sometimes spurious or malicious grounds.
In January, Britain's solicitor-general Vera Baird — a lawmaker who acts as a government legal adviser — acknowledged loopholes in current laws allowed pressure groups to use the threat of arrest to intimidate some foreign leaders, including Israelis.
The exact nature of the reform hasn't yet been made clear. The Ministry of Justice said only that a ministerial statement outlining the reform would be tabled later Thursday.
But Britain's Parliament would have to approve the rule change before it became law. Human rights groups and British Muslim organizations have already voiced their opposition to any change.
Benjamin Ward, Human Rights Watch's deputy director for Europe, said his organization "would be concerned with any proposal that would effectively abolish private prosecutions for these crimes."
A similar tightening of the rules has already taken place in Spain, which like Britain also subscribed to the concept of universal jurisdiction — and has also attracted Israel's ire.
Spanish judges have indicted Chilean dictator Augusto Pinochet and Al-Qaeda leader Osama Bin Laden, among others. The practical effect of the doctrine was negligible because extraditions were vanishingly rare — but momentum for reform started building last year after a judge started probing a deadly 2002 Israeli air force bombing and alleged Chinese abuses in Tibet.
Both Israel and China complained, and Spanish officials promised that the government would amend the law. In a rare show of bipartisan unity, the rules were changed last year so that they could only be invoked if Spanish victims were involved, or if the alleged perpetrators were in Spain.
Other European countries such as France, Belgium and the Netherlands have variants of the universal jurisdiction laws, but prosecutions have a checkered record.
There was no immediate comment from Israel's foreign ministry to Brown's announcement Thursday.
