Guy Burgess’s briefcase among MI5 artefacts on display

A battered leather briefcase left behind by Guy Burgess when he fled to Moscow in 1951 is among 20 objects from MI5’s archives to go on display for the first time from Saturday.

A joint exhibition with the National Archives tells the story of the first decades of the Security Service through objects and documents.

MI5 Director General Sir Ken McCallum says the exhibition shows “commitment to being open wherever we can”.

MI5 has sent 6,000 paper records to the Archives since 1997 but chooses what it will release, unlike most parts of government.

The exhibition includes a blackened, withered lemon, 110 years old, which formed part of the evidence against Kurt Muller, caught spying for Germany in 1915.

He had used lemon juice to add secret messages to letters written to Holland. When arrested, Muller had the lemon in his pocket.

He was tried, secretly found guilty and executed at the Tower of London in June 1915.

MI5 then mounted its first deception operation, continuing to send letters to the Germans, in Muller’s name, with false information.

With the money the Germans sent to the false Muller, the Security Service bought a new office car – for which the exhibition says they were “reprimanded” by the Treasury.

One of the most striking exhibits is a fake Nazi medal, made for one of MI5’s most successul wartime agents.

Eric Roberts, known as Jack King, had posed as a Gestapo agent, to find and work with Nazi sympathisers in Britain.

He identified nearly 500 people, and gave the Nazi “War Merit Cross” to two of them, who thought they were being rewarded for their wartime service to the German Reich.

As the BBC revealed in 2014, later became disillusioned with the Service.

The Cold War section includes one of the two briefcases left by Guy Burgess at his London club, when he fled to Moscow in 1951 with fellow spy Donald Maclean. It is marked with his initials.

What MI5 found in the briefcase is not included, although Burgess’s passport is on show.

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 spy Donald Maclean in 1951 due to fears of being discovered.

None of the Cambridge Five were ever prosecuted for spying.

MI5 files released to the National Archives date back decades and there are few exhibits from the more modern era.

One is part of a mortar from 1991, when the Provisional IRA attacked Number 10 Downing Street.

A Cabinet meeting was taking place as a bomb exploded outside. No one was hurt.

The exhibition caption explains that MI5 had advised Downing Street to replace the windows with laminated glass, as they had “monitored PIRA weaponry and bomb technology”.

“Had it not been for the windows,” an MI5 insider reflected, “the place would have been shredded.”

Sir Ken McCallum said the security service had been discussing a possible exhibition for several years.

Mark Dunton, National Archives historian and curator of MI5: Official Secret, said: “We were aware how interested the public are in the whole world of espionage.

“We suggested [the exhibition] first but MI5 thought about it and they thought ‘wow yes this would be a good thing.'”

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