Mouldy lemon among MI5 artefacts to go on display for first time

Telegraph

02-04-2025

A shrivelled lemon at the centre of a First World War spy drama is among dozens of items throwing new light on the secret history of MI5's battle against this country's enemies.
Now brown and desiccated, the 110-year-old lemon was central to a German spy plot to undermine Britain's defences during the 1914-18 war.
The lemon was found by an MI5 agent in the bureau of the Bloomsbury lodgings of German spy Karl Muller, who had been using its juice to write invisible letters to his paymasters, detailing the movement of British troops along the south coast in 1915.
When the letters were intercepted by the Postal and Telegraphic Censorship Department, MI5 ran a warm iron over them to reveal the writing left by the lemon juice.
Muller claimed to have been using the lemon to clean his teeth, but the fruit was presented as key evidence against him at his Old Bailey trial, following which he was executed at dawn by firing squad at the Tower of London.
When the Germans continued to send funds to Muller, before realising his cover had been blown, MI5 used the money to buy a two-seater Morris car – earning the security service a rebuke from the Treasury for misuse of public funds.
The surviving remnants of the lemon have gone on display at the National Archives in Kew, along with several previously top-secret documents and artefacts revealed to the public for the first time.
The MI5: Official Secrets exhibition at the National Archives in Kew leads viewers through the history of the service since its founding in 1909 at the height of a public panic over German spies.
At first, it was manned by just two officers, Captain Vernon Kell and Commander Sir Mansfield Smith-Cumming – and a typist.
The service quickly divided in two – MI5 and MI6 – to cover the threats from both domestic subversives and foreign espionage, with Cumming's initial 'C' adopted as the designation for all subsequent MI6 chiefs.
Soon, dozens of women were employed by MI5 on the vital task of amassing the registry of index cards on which the names of suspects and persons of interest were documented, while a network of agents, male and female, was established in the field.
Among the many plots they went on to foil was the Portland spy ring, whose Soviet operatives Lona and Morris Cohen (under the names Helen and Peter Kroger) transmitted top secret information to Moscow from their bungalow in Ruislip, north-west London.
A tin of Yardley talcum powder containing a secret micro dot reader and film used by the plotters between 1953 and 1961 is on display at Kew, as is the radio equipment buried in the home of the Krogers.
Also on display is an internal MI5 memo from March 1973, which records the reaction of the Queen on being informed that Sir Anthony Blunt, the surveyor of her paintings from 1945 to 1972, had been a Soviet agent all along and the fourth member of the Cambridge Five spy ring, which also included Guy Burgess, Donald Maclean and Kim Philby.
In the memo, displayed for the first time, an MI5 officer states: 'She took it all very calmly and without surprise; she remembered that he had been under suspicion way back in the aftermath of the BURGESS/MACLEAN case. She has been told that the danger of publicity would be quite strong after BLUNT's death.'
In fact, the scandal resurfaced in 1979, ahead of Blunt's death, when his confession, initially made in April 1964 in the study of his apartment in Portman Square – faithfully recreated by the exhibition – was revealed publicly by Prime Minister Margaret Thatcher.
Evidence of MI5's surveillance of Britain's growing fascist movement during the 1930s is also displayed, including the British Union of Fascists armband worn by Mitzi Smythe, a German woman who ran a boarding house in Ramsgate and was imprisoned alongside Oswald Mosley and his wife Diana.
The ongoing threat confronted by MI5 is illustrated by two more recent items.
One is the heavy tubular mortar fired by the IRA at Downing Street on Feb 7 1991. Acting on advice from MI5's C branch, which had been monitoring the IRA's weaponry, the windows had only recently been replaced with reinforced laminated glass to withstand mortar attacks, saving those inside from injury.
The other is a soft drink bottle, innocent-looking save for being strapped to an instant camera to form an explosive device.
This was one of several bombs Al Qaeda planned to use to blow up seven flights leaving Heathrow for the United States and Canada in the summer of 2006.
Operation OVERT, the largest counter-terrorist operation in the history of MI5 and the Metropolitan Police, managed to successfully disrupt the deadly plot.
Speaking about the exhibition, Sir Ken McCallum, the Director General of MI5, said: 'We've been protecting the UK from the most serious threats to our national security for 115 years.
'That headline mission, and the values that underpin it, haven't changed much. But how we keep the country safe is always evolving, always dynamic, always fascinating.'
He added: 'MI5 life is about ordinary human beings together doing extraordinary things to keep our country safe. I pay tribute to the teams doing that work today, right now. I equally pay tribute to our dedicated, often ingenious, predecessors.'