If ever the Second World War presented a quiet moment, it was not May 1944. The Russians recaptured Crimea, the Allies made a breakthrough in Italy, and Churchill and Eisenhower were thrashing out the final details for D-Day. Yet somewhere in the ranks of MI5, one officer was stubbornly refusing to alter his routine. While his colleagues were busily preparing for the Normandy landings, he was maintaining an admirable commitment to The Daily Telegraph crossword.

On May 2, 70 years ago this Friday, his puzzle was not the distraction he might have sought. Among the usual teasers, the clue for 17 across – “One of the U.S.” (4) – appeared routine. But the officer knew that the answer, Utah, was a codeword for one of the beaches to be used in the landings. This could be dismissed as coincidence, but three weeks later “Omaha” (another D-Day beach) appeared as a solution, followed in the next fortnight by “Overlord” (code for the entire operation), “Mulberry” (floating harbours) and “Neptune” (naval assault).

Within days, the crosswords’ compiler, Leonard Dawe, was being questioned by MI5. Propaganda posters had warned for years that the enemy lurked within, but the security service had never dreamt that the puzzles page of the Telegraph could be a harbour for subversives.

Dawe, who had been setting the crossword since its inception in 1925 and was now into his Fifties, was headmaster of the Strand School, which had been evacuated to Effingham in Surrey. Tom Weston, then head boy, still remembers the day MI5 arrived.

“An official-looking car turned up,” he recalls. “I was interested, so I kept watching. After a time, I saw Mr Dawe go off in the car with whoever it was.”

When the boys heard of the scandal, they were appalled. “We were astonished at the thought that Dawe was a traitor. He was a member of the local golf club. It was a complete mystery to most of us.”

Dawe did little to dispel the mystery when he returned to the school a few days later. He resumed setting crosswords, and said nothing at all about the incident for more than a decade.

Then, in a BBC interview in 1958, he described the ordeal. “They turned me inside out and collected naval intelligence. They went to Bury St Edmunds where my senior colleague Melville Jones [the paper’s other crossword compiler] was living and put him through the works.” Despite their suspicions, Dawe explained that the interrogators “eventually decided not to shoot us after all”.

It took another three decades for an apparent explanation to emerge. As part of the commemorations for the 40th anniversary of D-Day, the Telegraph revisited the crossword saga. Bill Deedes, then the paper’s editor, was alarmed by the scandal afresh, and instructed the puzzles editor to check that no codewords relating to the Falklands had appeared in the crossword during the recent conflict. None was found.

A few days later, Ronald French, another Old Strandian who had been encouraged by the renewed interest, wrote to the paper to admit to inserting the clues himself. Dawe, it emerged, would invite his pupils to fill in his blank crosswords with any words that came to mind. He would later devise clues to match the boys’ solutions.

With the war at its height, the excitable teenagers were obsessed by the vocabulary of the era, which is why other solutions of the time included “warden”, “Poland”, “aircraft” and “disarm”.

Likewise, the codewords were no coincidence. US and Canadian soldiers preparing for D-Day were camped close to the school, and the boys would regularly mix with them.

“The soldiers were obviously lonely,” recalls Bryan Belfont, a year below French. “Many had children of their own, and they more or less adopted us. We’d sit and chat and they’d give us chocolate.”

It was during one of these conversations that French heard the codewords. Security was remarkably lax, and he had struck up close friendships with the soldiers, regularly taking the colonel’s dog for a walk and even, on one occasion, driving a tank.

“Everyone knew the outline invasion plan and they knew the codewords,” he explained. “Omaha and Utah were the beaches, and they knew the names but not the locations. We all knew the operation was called Overlord.”

Perhaps to show off his knowledge, he slipped these words into the crossword. He bitterly regretted it, however, once he learnt of the trouble he had caused.

“Soon after D-Day, Dawe sent for me and asked me where I had got the words from. I told him and he asked to see my notebooks. He was horrified and said that the books must be burnt at once.

“He then gave me a stern lecture about national security and made me swear that I would tell no one about the matter. I have kept to that oath until now.”

Even French’s son, Simon, knew nothing of the affair until his father wrote to the Telegraph. “At the time, it was quite a scare for him,” he says now. “He was genuinely worried about what might happen, and whether he would cost the headmaster his job. But when it came out, he was quite proud of being involved.”

Ronald French retained his youthful enthusiasm for words, and continued to complete the Telegraph’s crossword every day until his death a few years ago.

One riddle remains, however. Nearly two years before the D-Day affair, on 18 August 1942, “Dieppe” was one of the paper’s crossword solutions. A day later, a disastrous raid took place on the port, with 3,623 of the 6,086 men who made it ashore killed, wounded or captured.

At the time, a War Office investigation concluded that the incident was “a remarkable coincidence”. Given what we now know about the later episode, this judgment seems open to question.

Perhaps one day someone will figure it out. For now, all these years later, Dawe’s crosswords remain cryptic.

The D-Day crosswords have been collected together for the first time in a new book of 100 classic Telegraph crosswords chosen by our Puzzles Editor, Phil McNeill. The Telegraph Centenary Crossword Collection (Hamlyn) is available for £9.99 + £1.10 p&p: please visit books.telegraph.co.uk or call 0844 871 1514.