Recently, an intruder was arrested inside Windsor Castle at Prince William’s 21st birthday party, another in a long line of Royal security breaches. Experts are baffled by how the man managed to evade a tight police cordon and allegedly climb on to the stage at the event as the Prince was making a speech.

Security at official Royal residences and at external events has been beefed up over the years but intruders have still been able to get surprisingly close to senior members of the Royal family.

The most serious breach came in March 1982 when Michael Fagan broke into the Queen’s bedroom at Buckingham Palace. She woke to find him sitting on her bed. The pair allegedly chatted for half an hour. Fagan, who was 30 at the time, was later jailed.

But the incident was by no means the only security breach to shake the Royals. In 1974, a deranged gunman tried to abduct Princess Anne as she and her first husband Captain Mark Phillips were being driven along the Mall to Buckingham Palace after a charity film show.

The would-be kidnapper, Ian Ball, forced the royal car to a halt and brandished a pistol at the driver and bodyguard. The kidnap bid was thwarted.

Anne’s personal protection officer James Beaton, shot and wounded during the incident, was later awarded the George Cross for his bravery. Two months later, Ball was sent to a mental hospital by a judge at the Old Bailey.

In 1979, Earl Mountbatten of Burma, great-uncle of the Prince of Wales, was killed when an INLA bomb blew apart his fishing boat off Mullaghmore, Co Sligo, where he had a holiday home.

In 1981, Marcus Sarjeant, 17, fired six blank shots at the Queen at the Trooping the Colour ceremony, reports Daily Mail.