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Various air assets including refuelling tankers were also seen on flight trackers moving from Europe, with reports of F-22 and F-35 strike aircraft following.

Also, he believed that the refuelling equipment used before departure "has likely been quarantined and already inspected".

Spain's Policia Nacional said the gang used 11 so-called narco boats to smuggle the drugs across the Atlantic in a complex operation that involved using an abandoned wreck at sea as a refuelling platform.

It could require years to fit with additional security systems and upgrades required to carry the president - including the ability to withstand the electromagnetic pulse from a nuclear blast, and to refuel mid-flight.

The aircraft would have to be made capable of in-flight refuelling and retrofitted with a sophisticated package of communications and security equipment.