Olympic silver-medallist Louis Smith has revealed on Twitter that he turned up at Buckingham Palace without his invitation.
The gymnast, who trains in Basildon in Essex, blamed his manager for forgetting it.
Smith will be presented with an MBE today.
Today is the day I become Louis Smith M.B.E ooooooo exciting times
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The plaque at number 16 Cowley Street in Westminster marks the brown brick Georgian townhouse where Sir John lived for 31 years.
A man reportedly armed with several knives has been arrested in Whitehall on suspicion of planning a terror attack, police have said.
Metropolitan Police officer, Tom Harrison, has been crawling the course since the race began on Sunday morning.