Lemon for invisible ink, spy's briefcase and IRA mortar bomb on display in unique MI5 exhibition

1 April 2025, 18:29 | Updated: 2 April 2025, 04:10

A passport belonging to one of the Cambridge spies, a 110-year-old lemon used for invisible ink and a letter about the Queen's response to news of a Soviet agent in Buckingham Palace are among MI5 artefacts on display in a "groundbreaking" new exhibition. 

MI5: Official Secrets features declassified documents alongside objects from the agency's private collection - many of which have never been seen before.

It marks the first time the intelligence agency has ever collaborated to display its files to the public.

A leather briefcase left at London's Reform Club by Cambridge spy Guy Burgess as he fled to Moscow in 1951 is one of the items on display.

His British passport is also on show for the first time.

Read more: The spies that betrayed Britain - the Cambridge Ring

Mr Burgess was a British diplomat and Soviet double agent during the Second World War and the early Cold War period.

He was a member of the Cambridge Five spy ring and fled to Moscow with fellow traitor Donald Maclean due to fears of being uncovered.

Another member of the ring was the late Queen's art adviser, Anthony Blunt. Included in the exhibition is a note confirming that her private secretary had told her about Blunt's treachery.

It says the Queen reacted "very calmly and without surprise".

None of the Cambridge Five were ever prosecuted.

A 110-year-old lemon is another of the objects displayed and was a key piece of evidence used against German spy Karl Muller, who was executed by firing squad at the Tower of London in 1915.

Muller used lemon juice as invisible ink to inform on British troop movements. A warm iron was passed over a letter to reveal the secret messages.

The lemon was found in his overcoat when he was arrested.

Other items loaned include MI5's first camera, a key to the Communist Party of Great Britain's Westminster branch office, and a Provisional Irish Republican Army (PIRA) mortar bomb.

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Mark Dunton, principal records specialist at the National Archives, said the exhibition is "genuinely groundbreaking".

"MI5 used to really operate in secret, for so many years it was just referred to as PO Box 500 - really anonymous.

"But once we got into the 1990s, it became more and more of an open organisation - the identity of the director general was revealed in 1992 publicly, and in 1997, MI5 began transferring files to the National Archives."

MI5: Official Secrets is running from 5 April until 28 September at The National Archives in Kew, London. Admission is free.

(c) Sky News 2025: Lemon for invisible ink, spy's briefcase and IRA mortar bomb on display in unique MI5 exhibition