It can be only a matter of time before a prominent Israeli is arrested in London for alleged war crimes.

In the latest example of "lawfare" ? defined by the military lawyer Charles Dunlap in 2001 as "the use of law as a weapon of war" ? lawyers for the Palestine Solidarity Campaign asked a district judge to issue a warrant for the arrest of Ehud Barak, the Israeli Defence Minister. Barak, visiting Britain at the end of September, was accused of war crimes during Israel's offensive in Gaza at the end of last year.

For obvious reasons, a suspect is not told that a judge is considering his arrest. But the Foreign Office was alerted this time and Clare Montgomery QC was briefed to address the court. District Judge Daphne Wickham said the allegations against Barak had been well documented. But, she continued, "I am satisfied that under customary international law, Mr Barak has immunity from prosecution as he would not be able to perform his functions efficiently if he were the subject of criminal proceedings in this jurisdiction."

In 2002, the International Court of Justice ruled that a serving foreign minister has immunity from criminal jurisdiction while travelling abroad. Barak is a defence minister. But campaigners had attempted to have one of his predecessors, Shaul Mofaz, arrested on a visit to Britain in 2004. On that occasion, the judge District Judge Christopher Pratt noted that the international court had referred to the immunities of "certain holders of high-ranking office in a state, such as the Head of State, Head of Government and Minister for Foreign Affairs". The judge was persuaded that a defence minister would have the same immunity as a foreign minister.

In legal terms, this is not much of a precedent ? though it perhaps explains why Israel's Deputy Foreign Minister, Danny Ayalon, felt able to visit London in October.

But District Judge Pratt thought it unlikely that immunity would extend to ministers with wholly domestic responsibilities, such as culture, media and sport. This may be why the senior Israeli cabinet minister Moshe Ya'alon cancelled a planned trip to London the same month. He had been warned he faced possible arrest over his involvement in the assassination of a Hamas leader in 2002, in which 14 other people died.

And this is no idle threat. In 2005, a warrant was issued for the arrest of Doron Almog, a former general in the Israeli army. Almog never left the plane that brought him to London. A message from the Israeli embassy warned him that officers were standing by to intercept him at the immigration desk. How did the embassy know? A leaked police report says that the police Special Branch raised the possibility of informing the Israeli ambassador's protection officers. Detective Superintendent John MacBrayne, from Scotland Yard's counter-terrorist command, told Special Branch any decision to do so would be a matter for them. This seems the most likely conduit.

The warrant for Almog's arrest was issued under the Geneva Conventions Act 1957, which was passed, somewhat belatedly, to allow Britain to ratify the four Geneva Conventions of 1949 for the protection of victims of war. Our courts have "universal jurisdiction" under the Act: under English law, it is an offence for anyone to commit a "grave breach" of one of the conventions.

There is an important safeguard against inappropriate use of the Geneva Conventions Act. English law says that criminal proceedings in respect of alleged offences occurring after August 2001 cannot be instituted without the Attorney General's consent.

But that safeguard does not extend to arrest. Anyone may obtain a warrant from a magistrate without notice by producing information, substantiated on oath, that a named person is suspected of a serious offence. Broadly speaking, the police must then arrest the person concerned.

At one stage, it looked as if ministers were willing to amend the law by giving the Attorney General a veto over arrests in Britain for war crimes allegedly committed abroad. But the government has failed to make good that pledge, despite a recent legislative opportunity. It should now do so. There should be no power of arrest in cases where there is no realistic prospect of a trial. And no Attorney General would consent to a prosecution that would force the only democracy in the Middle East to withdraw its diplomatic relations.

Unless the law on arrest is changed, Britain will lose any remaining influence it still has as an honest broker in the region. And the next generals to find themselves victims of lawfare in Britain may be visiting Iraq war veterans from the United States.