Police and spooks in charge of security for tomorrow's royal wedding have planned for every possible eventuality - including that of Kate leaving Wills at the altar.

The top-secret contingency plan for a "runaway bride" scenario has been dubbed "Operation Pumpkin", and if put into effect would see hundreds of operatives switch tasks in a desperate attempt to generate a moving security cordon around the escaping future Queen - while simultaneously attempting to preserve Prince William's option to pursue and dramatically win her back.

"Frankly, it's a nightmare scenario," one highly-placed MI5 source confessed to the Reg yesterday. "But you have to plan for every possible contingency, and let's face it, this one's hardly that unlikely, is it? Obviously we had to get together with Clarence House and work something out.

"Naturally, they wanted us to simply make her go through with it - but we said you must be joking: in the UK, in front of millions of witnesses? No way. And the plods would never play ball anyhow.

"So we said, look, if she bottles it we'll just have to get her out pronto, helicopter off the roof maybe, then sort her out with a new identity and young Bill will just have to go on the honeymoon on his own."

However our source said that royal officials were unhappy with that plan. They reluctantly accepted that Ms Middleton remains free to bolt right up until the last moment, but argued that she should be compelled to flee on foot for at least a short distance, so allowing the Prince to pursue her and so perhaps regain some PR benefit for the royal family - and maybe even persuade the absconding future consort to come up to scratch once more.

"That's when it got difficult," says our source. "Now you've got her running out of the Abbey, crowds everywhere, him chasing after her. She's got to be able to run, he's got to be able to catch up if he can. Nightmare."

After protracted, top-secret negotiations between royal staff from Clarence House and representatives from the Metropolitan Police, MI5 and elements of the military, a compromise was agreed. In the event of Operation Pumpkin being put into effect Ms Middleton will be permitted to run out of Westminster Abbey with her bodyguards trailing discreetly at a distance. Plain-clothes undercover police, MI5 officers and SAS soldiers stationed in the crowd will form a mobile flying wedge ahead of her, clearing a path for the fugitive future princess to escape down.

Prince William will then have a limited time, the subject of tense negotiations between Clarence House and security chiefs, in which the path behind Ms Middleton will be kept open for him to go after her, after which the mobile protective cordon will close again at the Abbey end due to lack of manpower and the Prince will have let his bride slip through his fingers.

If Wills reacts fast enough, however, he will be able to chase after his fleeing fiancee for just under half a mile.

"Clarence House wanted a full mile," says our source. "But we said come on, play fair, she's in her wedding dress and there has to be some limit on the overtime budget."

If the Prince fails to intercept Ms Middleton over that distance, the security team will decide that no on-the-spot reconciliation is possible and a strategically positioned taxi, driven by an undercover SAS operative and unobtrusively escorted by several unmarked police cars, will opportunely pull up to carry the escaping ex-future-princess to safety.

On the other hand if Wills manages to come up with Kate he will be allowed to attempt to persuade her to return with him for a limited period.

"We got Clarence House to cut that by a few minutes by agreeing they could put ringers in the crowd to shout stuff like 'Go on love, give him a kiss'," reveals our source.

It is understood that royal officials are hoping that there will be no need for Operation Pumpkin, but anticipate a significant boost to the wedding's TV ratings if it does go into effect.

We asked our source what plans were in place should Wills, rather than Kate, attempt to flee the wedding.

"Come off it," he said. "We only plan for things that make sense. He doesn't want to be back on the dating scene wearing a rug, does he?" ®