Senior ministers are discussing how Britain would respond in the event of a military confrontation between Israel and Iran later this year.
With new international talks about Tehran's nuclear programme due to start in Baghdad later, ministers remain hopeful that diplomacy can prevent Iran developing a nuclear weapon and persuade Israel not to carry out its threat to bomb Iranian nuclear facilities.
However, I have learnt that UK ministers are discussing not just the possibility of a military confrontation but what role, if any, Britain might play and whether any involvement would be legal.
Last week in London, the National Security Council discussed what would happen if the latest set of negotiations with Iran failed and if Israel carried out its threat to launch a military strike on Iran's nuclear facilities.
Ministers were told that this could trigger a wider war in the Middle East in which Iran might respond not just by attacking Israel, but also by closing the vital trade route - the Straits of Hormuz - through which more than a fifth of the world's oil shipments are carried.
I understand that the government's law officers are now examining the legality of any British involvement if this happens.
They are looking at options ranging from British diplomatic support for Israel through to the possible involvement of the Royal Navy in the region.
The Liberal Democrats, who opposed the invasion of Iraq, insist that in future Britain must only do what is clearly within international law.
Ministers are still placing their hopes in what Churchill called "jaw jaw" but, behind the scenes, they are discussing in detail how they might respond to "war war".
Later on Wednesday, a new series of Decision Time on BBC Radio 4 (at 20:00 BST) examines how the British and American governments would approach a decision on how to prevent war in the Middle East and what involvement to have if it occurs.
One decision that would face the Coalition if war breaks out between Iran and Israel is the role of Diego Garcia. It is a British colony in the Indian Ocean but has for many years acted as a US base and island aircraft carrier.
B52 and B2 bombers were used to attack Iraq and Afghanistan and could be used to attack Iran if the United States decided to join forces with Israel.
The UK leases Diego Garcia to the US and the terms of that lease have, I'm told, never been published. One question for the government's lawyers is whether the British government would have to give it's permission for Diego Garcia to be used in any attack on Iran.
All these decisions would not unduly trouble a Tory government which decided to back the USA and Israel. However, they could prove to be a Coalition breaker.
Labour has given its reaction to the discussions. Shadow foreign secretary Douglas Alexander said: "Instead of sabre rattling in public about what could happen if the diplomatic efforts fail, the government should be focussed on ensuring a successful outcome to the talks now underway. Now is the time both to be engaging directly with Iran and increasing the diplomatic pressure upon Iran to meet its obligations under the NPT."