A shadowy gathering of spies and codebreakers in 1938, under cover of a country house shooting party, has gone down in history as being a rehearsal for the real work of Bletchley Park, but research has revealed a more intriguing story.

The men and women from MI6 and the government’s code and cipher school (GC&CS) were sent there as part of a much bigger and more serious operation than previously thought.

“This wasn’t simply a recce or friendly rehearsal,” said Bletchley Park’s research historian, David Kenyon. “It was the activation of one of the government’s most secret war stations at a time when conflict appeared imminent.”

The details are being revealed to coincide with an important anniversary. It was 80 years ago on 18 September that men and women arrived at the Buckinghamshire country mansion as guests of what became known as “Captain Ridley’s shooting party.”

It had been thought that about 50 people were involved. Kenyon said the figure was more like 150, all members of MI6 and GC&CS, the forerunner of GCHQ. Kenyon said the real urgency of their work had only recently been understood.

At the time, Germany’s Enigma code was unbroken but progress had been made with Italian navy codes, which is where most of the 1938 work was directed. That also served a political purpose in that the government saw Italian actions as being more potentially harmful to British interests than German ones. “The idea that the Italian navy might start getting belligerent in the Mediterranean is in many ways a bigger strategic threat than whatever might happen in Czechoslovakia.”

While the 1938 visit was not a recce, valuable lessons were learned to ensure Bletchley was better equipped when the work there began in 1939, not least the need for bookshelves and filing cabinets.

Bletchley Park had been bought for the nation by Admiral Hugh Sinclair, or C, director of GC&CS and SIS, in May 1938. Locally, tongues wagged from the outset with rumours of it being connected to the air defence of London should a war break out.

During the shooting party, gossip went to a different level when it was noticed how many older men and young female secretaries were booked into local hotels and pubs.

The head of the naval section, “Nobby” Clarke, aged 55 in 1938, recalled: “It seems to have been thought that this was a party of elderly gentlemen with their young women.

“A chambermaid at the hotel who was complaining of overwork, on being told times were serious and that she should not complain, said: ‘It’s all right for you but some of us have to do work.’ Little did she realise what these odd people were doing.”

The wartime work at Bletchley Park is estimated to have shortened the war by two years and saved thousands of lives.