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”.