Expert: "I do believe we will bring one down."
Nick Pope — who worked on the Ministry of Defence's UFO desk for three years — revealed the rules of engagement for the first time.
He claimed RAF pilots had fired at UFOs on several occasions — but failed to bring them down.
He added: "We know of cases where the order has been given to shoot down — with little effect to the UFO."
Mr Pope said the rules of engagement were drawn up after dozens of close encounters with suspect craft in British airspace.
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He added: "I do believe we will bring one down. We're developing increasingly sophisticated weapons."