It has long been rumoured, and often reported, that in return for bankrolling the Pakistani nuclear weapons project, Saudi Arabia has a claim on some of those weapons in time of need. It has never been proved though, nor has it ever been clear how such a deal would work.

BBC Newsnight had a stab at those questions last night, in a report saying that Saudi Arabia could obtain nukes "at will" from Pakistan. Here are the key paragraphs:

Earlier this year, a senior Nato decision maker told me that he had seen intelligence reporting that nuclear weapons made in Pakistan on behalf of Saudi Arabia are now sitting ready for delivery. Last month Amos Yadlin, a former head of Israeli military intelligence, told a conference in Sweden that if Iran got the bomb, "the Saudis will not wait one month. They already paid for the bomb, they will go to Pakistan and bring what they need to bring."

The "Nato decision-maker" bit looks interesting but is somewhat undermined a bit later in the TV version by the revelation that the intelligence is thought to have originated in Israel.

The trouble is lots of intelligence reports originate in Israel and some are probably true, but the timing of this one, while talks on the Iranian nuclear programme are underway, is fairly convenient. The narrative of the nuclear cascade triggered by the Iranian programme is familiar. It is cited as the reason Iranian progress must be halted at all costs. It also serves as a deterrent to Iran.

Mark Fitzpatrick, who has followed this story through the years as a non-proliferation expert at the state department and now at the International Institute for Strategic Studies had this to say:

It is not a new story, of course, but Urban came up with some new data points: a Saudi belief that it could obtain nuclear weapons from Pakistan at any time, and reported intelligence that Pakistan has prepared nuclear weapons for delivery to Saudi Arabia. The first part is probably true: The Saudis helped to finance Pakistan's nuclear weapons programme and believe that they were given a promise that the weapons would be used to defend the Saudi kingdom if need be. The second part is probably false: I doubt that Pakistan is ready to send nuclear weapons to Saudi Arabia. Pakistan's reputation suffered greatly the last time they assisted other countries with nuclear weapons technology (i.e., the sales by A.Q. Khan, with some governmental support or at least acquiescence, to North Korea, Iran and Libya). Pakistan knows that transferring nuclear weapons to Saudi Arabia would also incur huge diplomatic and reputational costs. Also, Saudi Arabia has no need today for nuclear weapons. It can wait to see if Iran's program will be stopped through diplomatic means. That said, Saudi Arabia's foreign policy is undergoing some significant shifts, as evidenced by the decision not to accept the UN Security Council seat, so the situation has become more unpredictable. It is conceivable that Saudi Arabia planted some of the evidence for this story as a means of putting pressure on the United States to be firm in dealing with Iran.

David Albright, the head of the Institute for Science and International Security, broadly agrees. This was his comment:

Would Pakistan give them [nuclear weapons]? There would be real punishments for that and they would want to avoid those. For Saudi Arabia to take possession it would mean withdrawing from the NPT [nuclear Non-Proliferation Treaty]. Any US military sales would have to stop. That could not be ignored. Only in a very dire situation in which Iran has a nuclear weapon and is being confrontational, could you imagine something like this.

This sounds about right to me. If the Saudis are constantly calibrating the costs and benefits of shipping in Pakistani nukes, I imagine they are still a long way from calling in their chit, and the Pakistanis likewise. But it serves both countries, as well as Israel, to have the story front and centre while US and Iran sit down in Geneva.