Buckingham Palace was “locked down” five days after the July 7 London bombings in 2005.

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Peter Scott, former head of Scotland Yard’s Counter terrorism Command, has revealed fears of a fifth suicide bomber caused panic in the palace, as well as the Houses of Parliament and Scotland Yard itself.

Queen Elizabeth is believed to have been in the royal residence hosting an official reception with Prince Philip, Prince Andrew, Princess Anne, and Prince Edward’s wife the Countess of Wessex when the building was sealed off for 90 minutes on July 12.

CCTV footage led police to believe there was a fifth bomber on the loose and prompted the drastic action.

Speaking at the inquest of Charles de Menezes – who was killed by police officers who suspected him of being a terrorist in 2005 – Scott said: “It was completely unprecedented, as was some of the decision making having to be made at that time about whether or not to warn the public about the possibility of a fifth suicide bomber.

“If you warn the public you could cause unnecessary panic, if you don’t and something terrible happened, the obvious question is why didn’t you warn the public?

“That’s the sort of pressure we were under day in, day out. July 12 is but one example.”