After spending nearly 35 years as a public prosecutor, it can?t be very easy to switch from advising on what the law is to thinking about what it ought to be.

That?s the problem facing Chris Newell, 58, who is speaking to me less than 12 hours after retiring as principal legal adviser to the director of public prosecutions. Even though Newell is no longer on the payroll, a press officer from the Crown Prosecution Service is monitoring our every word. But it?s hardly necessary: such a consummate professional would never let down the service for which he has worked since its inception in 1986.

During the previous decade he divided his time between working for the director of public prosecutions ? who then ran a small, self-contained London office of just 80 lawyers ? and for the attorney general, whose staff was smaller still. I distinctly remember Newell?s discomfort as he was required to convey some gentle rebuke to me from a government law officer who thought that I might be in contempt of court; he, though, has generously forgotten.

Only once during our hour-long interview do I lure this cautious barrister into hinting that he might favour a change in the law. We are discussing an anomaly which, though little known, has caused great offence to a friendly foreign power.

The story begins in September 2005 when the father of a severely disabled child landed at Heathrow Airport. He was en route to Solihull, where he was to speak about a project in the Negev that helps Israeli Jews and Arabs with severe mental and physical disabilities.

The man was Doron Almog, a retired general who was head of the Israeli Defence Force?s Southern Command from 2000 to 2003. A message had reached Major General Almog from his country?s embassy, warning him that a district judge at Bow Street had issued a warrant for his arrest and that police were standing by. Almog returned to Israel without ever leaving the plane.

Incidentally, nobody has ever established who tipped off the embassy. I strongly suspect it was Scotland Yard, whose officers feared the ?international impact of a potentially armed police operation at an airport?.

What we do know is that Almog was alleged to have ?wantonly? destroyed 59 houses in Rafah, on the border between Gaza and Egypt, in January 2002. Israel firmly denied any breaches of the laws of war and Almog himself has denied wrongdoing.

The arrest warrant 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. Section 1 of the act makes it an offence under English law for anyone to commit a ?grave breach? of one of the conventions.

These include ?extensive destruction and appropriation of property, not justified by military necessity and carried out unlawfully and wantonly?.

There is, however, an important safeguard. Section 1A of the amended 1957 act says that criminal proceedings in respect of alleged offences occurring after August 2001 cannot be instituted without the consent of the attorney general. But that safeguard does not extend to arrest. Anybody can obtain an arrest warrant on the basis of sworn information.

As Newell recognises, this is an anomaly. ?There is an argument for saying that, if it?s an offence that requires the attorney general?s consent, should that not be a prerequisite to obtaining a warrant in the first place? Because there?s not much point in getting a warrant from the magistrates? court if the attorney is never going to give consent anyway,? he tells me.

?Forgive the colloquialisms,? Newell continues, ?but if it?s all going to end up in the bin, you could argue that the sooner it gets in the bin the better ? if only from the point of view of the public purse.?

Quite right. There was no prospect that Almog would have stood trial in Britain for offences alleged to have taken place in Israel. Quite apart from the difficulties in proving the allegations, such a trial would have led to a breakdown in diplomatic relations between Britain and a democratic country ? too democratic, some might say ? with which Britain has important trading links.

The point Newell makes is well understood by the Foreign Office. At one stage, it looked like ministers were willing to amend the law by giving the attorney general a veto over arrests for war crimes. ?Both the Blair and Brown governments promised us they would bring the necessary legislation before Parliament,? an unnamed Israeli diplomat told the Jewish Chronicle last month.

But the government is not currently seeking a change in the law. We know that because a government bill to amend the 1957 act, approved by the Commons last week, did not include a veto on arrests. The Geneva Conventions Bill, which gives the ?Red Crystal? equal status to the Red Cross and the Red Crescent, would have been an ideal legislative opportunity ? though the bill might not then have received such an easy ride.

So if anti-war groups can obtain detailed information about particular operations there is every prospect that, on some future occasion, the police will be ordered to arrest visiting US generals who have served in Iraq ? and perhaps British officers too.

Maybe the foreign secretary will order a review of the law, no doubt conducted by a former senior prosecutor who knows how the system works ? someone who has retired early to spend more time with his wife and young daughter. I don?t think Chris Newell is going to be putting his feet up just yet.