The family of a decorated British civil servant who built a notorious jail for the empire in Palestine has revealed he leaked the building plans to Jewish militants, helping them to launch a legendary prison break in 1947.

The storming of Acre prison has been credited as a critical event that led to the British decision to end what was increasingly viewed as an onerous mandate in Palestine. Until now, details of how the highly-sophisticated operation was so successful had remained elusive.

Gil Margulis, the great-nephew of prominent engineer and architect, Peres Etkes, believes he has the answer. “They actually had the plans of the whole prison from the guy who made it,” he told the Guardian.

“I was reading the history and people keep saying, ‘How did they do it? How did it happen’? Sometimes you need a little insider information. Well, they had a lot of insider information – they had the exact plans.”

An article by the Guardian filed late on 4 May, 1947, described a well-planned strike that day on the prison, which had been built on the ruins of a 12th century crusader fortress.

While protected by a moat on several sides, the article said, “Jewish terrorists” seized adjacent Turkish baths, one of the few sides of the prison, to break it. “It was from that vantage point that they succeeded in blowing a hole in the wall,” it added.

At the same time, militants threw a grenade into a separate part of the prison as a diversion and at least one of the attackers was disguised as a British Royal Engineer. “Jews and Arabs, terrorists and criminals, rushed out together,” wrote the paper, estimating around 250 fled.

Etkes, a Russian Jew and American citizen, was a prominent engineer working for the British forces. While helping London to establish Palestine as a strategic centre in the Eastern Mediterranean, the ardent Zionist’s true motivation was to develop what he believed would be the future state of Israel.

Fifty years since his death his great-nephew, Margulis, found Etkes’s half-written memoir while researching his life. In it, Etkes said he had also used his British connections in 1921 to transfer weapons from the British-run Jaffa armoury, which he then “lent” to Jewish forces in Tel Aviv during Arab riots.

While British forces found out he had transferred the rifles – and sent Etkes to the barren north as punishment – the incident was either forgotten or disregarded. Two decades later, he received the King’s Medal for Service in the Cause of Freedom for building a deep water port in the city of Haifa and for constructing a vast network of roads.

While Etkes’s memoirs end before the Acre prison break, he recounted the story during a 1950s visit to Brooklyn, to his niece and Margulis’s mother, Aliza. Etkes told her he had shared the plans “because the prison was like a fortress, and unless they had the map, there was no way to get out”.

Etkes also showed her a bible, she said, that had been personally signed by Winston Churchill and the war cabinet, thanking him for developing the Haifa port, which was seen a vital to the Allied war effort at the time.

But it was his involvement in the prison break that would perhaps prove to be the most consequential act of his career. It has long been seen as a dramatic symbol of London’s declining ability to maintain control in Mandatory Palestine, which ended a year later.

Menachem Begin, who led the Zionist Irgun paramilitary that claimed responsibility for the prison break, described it as the most important attack to have a “disintegrating effect on the [British] government’s prestige”. Begin later became Israel’ prime minister.

Gil Margulis says his great uncle saw no contradiction by working for both sides. “It wasn’t an anti-British thing,” he said. “I think the interesting thing is that, for a period, the empire and the Zionist movement were kind of walking in the same direction with different goals. For him, building the country was a big thing, and he was able to do that with the empire putting resources in.”

As for why he kept his involvement in one of imperial Britain’s most infamous prison breaks a family secret, Margulis said he did not want undue attention.

“For the rest of his life, he had a nice British pension. I think he said that he didn’t want to jeopardise that for a little news credit. It’s not mentioned anywhere for that very reason.”