At a special "review conference" in Kampala, Uganda, the nations which have signed the International Criminal Court (ICC)'s founding statute, including Britain, are considering a proposal to let the court try the "crime of aggression".
If the proposal, backed by more than 70 countries, passes, national leaders alleged to have launched "illegal" wars could be seized, transported to the Hague, tried and imprisoned. That would be the case even if such a leader was democratically elected.
UK law requires British police to enforce indictments and arrest warrants issued by the court.
Britain, a member of the court, is not against the plan in principle. But of course it is fighting furiously for safeguards that would protect Blair and future British prime ministers from arrest. Britain is understood to be fighting for a provision that no prosecution can be launched without the permission of the UN Security Council - giving the UK, as a permanent member of the council, an effective veto. However, other European and Latin American countries say there should be no such restriction.
The "crime of aggression" is included in the ICC's founding Statutes (1998 & 2002), as one of four crimes covered by the court. On the other three - genocide, war crimes, and crimes against humanity - the ICC already has the power to try individuals. It has indicted 14 people, three of whom are currently on trial in the Hague. Inexplicably, the Statute does not currently allow the court to prosecute cases of aggression. The Kampala proposal amends the Statute, providing a definition of the crime and bringing it within the court's jurisdiction for the first time.
But the Chilcot inquiry heard last week that Blair's own Foreign Office legal advisers unanimously believe that - in the absence of a second UN resolution - he did commit the crime of aggression. Even so, under the current ICC Statute Blair's actions in the Iraq war do not constitute a prosecutable war crime, genocide or crime against humanity.
Britain could refuse to ratify the amendment - or even withdraw from the court - if it decides that it is totally unacceptable, but that would be an embarrassment for a country that strongly supported the establishment of the ICC.
The US does not recognise the ICC and will not be able to vote at the Kampala conference, which takes place in June. However, it is expected to send a large delegation of observers to exert pressure against the amendment, which needs a two-thirds majority to pass.
Philippe Sands QC, professor of international law at University College, London, has commented,
I have heard very conflicting views about whether it will go through. Some people are optimistic and some are pessimistic.Salutary effects, too!
If it happened, it could concentrate minds in the UK and perhaps have very significant effects.