In the elegant drawing room, the pink gin is being served.

Newspapers and magazines are scattered over the coffee tables, while on the sofas the guests smoke cigarettes with their drinks as they await the sumptuous dinner to come.

The year is 1943, and although Britain and her allies are deep into a protracted war with Germany, there are no signs of deprivation or rationing here.

Quite the reverse. With billiards, art classes, cigars and fine wines on tap, the guests – all of them male – appear to be revelling in the trappings of life in a classic English country house.

Captured German generals were wined and dined in luxury, as in this scene from the 1966 film Triple Cross starring Yul Brynner

So much so that nobody witnessing this genteel scene could possibly guess the astonishing truth behind it: that these pampered guests are, in fact, high-ranking German prisoners of war responsible for countless deaths and atrocities – and that their luxurious surroundings are the setting for one of Britain’s most breathtakingly audacious and successful spying operations.

Only now, with the declassification of highly confidential documents and a raft of new research, can the details of one of wartime espionage’s best-kept secrets be fully revealed – along with the story of its brilliant spymaster, Thomas Kendrick, a charismatic ex-soldier whose character would not be out of place in a John le Carré novel.

Eighty years after this remarkable operation was first launched, I will describe how:

Kendrick arranged for captured German generals and field marshals to live a life of luxury in stately homes, while secretly recording all their private conversations.

Even the plant pots, billiard tables and light fittings were bugged in order to pick up nuggets of information that could help the British war effort.

More than 10,000 PoWs were eavesdropped on by teams of hundreds of ‘secret listeners’, many of them German emigres who had fled the Nazi regime.

German officers were flattered by their lavish treatment into revealing vital intelligence, boasting to each other about the stupidity of their British captors.

Kendrick ignored the orders of a furious Churchill to stop wining and dining prisoners at London’s most expensive restaurants.

Intelligence gathered by Kendrick and his staff led directly to raids which delayed the development of Germany’s long-range V1 and V2 flying bombs by months, allowing the D-Day landings to take place.

Cultured, quick-witted and a gifted pianist, Thomas Kendrick had already been a spy for 30 years as the Second World War was about to break out.

In 1938, he returned to London from a posting in Vienna to one of the biggest challenges of the conflict: as commander-in-chief of MI6’s brand new listening unit.

This ground-breaking and highly sophisticated operation, codenamed the M Room (M stood for ‘miked’), was a piece of early genius by British intelligence.

MI6’s idea was that prisoners of war would be matched up into groups of two or three and placed in cells together, where their unguarded conversations would be recorded, translated and analysed by teams of ‘secret listeners’ via hidden microphones.

It was a strategy which was to yield spectacular results, and Kendrick was the perfect choice to lead it.

Married to a German citizen and a fluent speaker of the language himself, he bore no ill-will towards his wife’s compatriots and refused to allow torture or cruelty by his staff.

Above all, he was a pragmatist whose sole aim was to find clever ways, however unorthodox, to get his prisoners to spill the beans.

The day that German forces invaded Poland, on September 1, 1939, Kendrick opened his first clandestine unit within a special compound at the Tower of London.

The historic site that had witnessed the deaths of Royals and traitors was about to find a whole new role within British history.

The first prisoners arrived at the Tower on September 17, 1939, from an enemy U-boat sunk off the coast of North-West Ireland. Others quickly followed.

Within weeks, more than 100 captives were held there. Mission M Room had begun.

As prisoners were brought in, they were subjected to a phoney interview, designed to give them a low impression of their questioners and avert suspicion that their accommodation might be bugged.

Back in their rooms, they would then brag to their fellow captives about how little they had revealed to their British interrogators.

One, Freiherr von Reitzenstein, who was picked up in the North Sea on October 1, joked with his cellmate after a five-hour interrogation: ‘My God, what s**t we served them up!’

Of course, the pair had fooled no one, Kendrick later writing that he had found them ‘crude, and incapable of deceiving anybody’.

Apparently trivial scraps of conversation between prisoners began yielding vital information.

Already an intelligence jigsaw was being built about Germany’s bombing capacity, radio communications, technical advances and military losses.

On November 9 came the first of many references to Hitler’s ‘secret weapon’, although it was unclear what this might be.

An M Room report at the time read: ‘Hitler’s secret weapon is talked about a lot. They [PoWs] believe in it, but say there is no possibility of bacteriological warfare.’

These conversations would later play a key role as the hostilities unfolded.

By the end of 1939, it was clear that the Tower of London could no longer cope with the massive influx of captives and three further centres – Trent Park at Cockfosters in North London, Wilton Park near Brighton, and Latimer House in the Buckinghamshire countryside – were opened.

When the cost of making the sites operational – about £400,000 then, the equivalent of £2 million today – was challenged by the Cabinet, the combined heads of the intelligence services responded in a no-nonsense memorandum.

They argued that the M Room unit was ‘of the utmost operational importance, vital to the needs of the three fighting services and should accordingly be given the highest degree of priority in all its requirements; that the normal formalities regarding surveys, plans and tenders should be waived; and that any work required should be put in hand at once and completed by the earliest possible date, irrespective of cost’.

From the prisoners had by now come vital information on U-boats, mines and torpedoes, the impact of air raids and details of underwater refuelling for warships.

The intelligence was invaluable. Yet still prisoners continued to believe they had outsmarted their interrogators.

A bomber mechanic captured in February 1941 told his cellmate in a conversation recorded by the secret listeners: ‘He [the interrogation officer] asked what engine the Dornier Do 217 had.

I said I don’t know. As a matter of fact I do know that it has the Bramo double-row radial engine.’

With the middle years of the war came the big prizes for Kendrick: captured German generals and field marshals, with their superior knowledge of military strategy and close links to Hitler. It was time to move the operation up a gear.

The first general to arrive at Trent Park, in August 1942, was the university-educated Ludwig Cruwell, who had succeeded Rommel as commander of the Afrika Korps, followed in October by 51-year-old General Ritter von Thoma, commander of a Panzer tank division and a veteran of Dunkirk.

And so began one of British intelligence’s most cunning deceptions of the war.

The generals had expected to be held in a rudimentary prisoner- of-war camp with Nissen huts and barbed wire – exactly the kind of surroundings that would have encouraged them to give little away.

Instead, they found themselves living in considerable comfort at what they believed was the generous behest of King George VI, as befitted their status as military commanders – a move that played superbly into their sense of self-importance.

Cruwell, Von Thoma and a third senior general, Hans Cramer, were given bedrooms and adjoining sitting rooms, with separate accommodation for their batmen.

A spacious area at Trent Park was set aside for creative activities – for painting and drawing, playing cards, table tennis, billiards, chess and bridge.

As they relaxed – and began dropping their guard – the generals devoted their days to learning languages and studying other subjects, while for those of a musical disposition a string quartet and grand piano were available.

They painted watercolours, had their own portraits painted and were taken to restaurants in London – even to Kendrick’s own home in Surrey.

So enamoured were they of their arrangements that one officer wrote home to his wife to say that he would love his family to be there with him.

‘Our involuntary hosts are thoroughly gentlemanlike,’ he said.

Little did they realise that everything that could be bugged was – from the light fittings to the fireplaces, plant pots, behind the skirting boards, under the floorboards of the bedrooms and even the trees in the gardens.

When they leaned out of a window to speak in order to avoid being overheard, they had no idea that the windowsills, too, were bugged.

The hidden microphones were wired back to the M Room in the basement, where teams of secret listeners worked from the moment the generals woke until the time they went to sleep again.

To flatter them still further, the generals’ every need was met by a man they were told was their welfare officer, Lord Aberfeldy.

There was, in fact, no such person – the so-called aristocrat was one of Kendrick’s intelligence officers, Ian Thomson Monro, a highly talented amateur actor.

‘A delightfully outgoing and intelligent Scot, he was the prototype of the officer and gentleman and his contribution to the war was to act this out to the full,’ said a fellow intelligence officer.

‘He took his guests on walks and to restaurants, galleries and shops in London, disarming not a few with his snob appeal and his assumed title.’

As Lord Aberfeldy, Monro provided a sympathetic ear for the generals and made special shopping trips once a fortnight to buy extra items they requested such as shaving cream and chocolates.

He even arranged for a tailor from Savile Row to come out to Trent Park to provide new clothes for them.

Eventually an exasperated Churchill, hearing about the prisoners’ regular lunches at Simpson’s in the Strand, banned the ‘pampering of the generals’.

But the intelligence services knew that the treatment of the generals was reaping excellent results that could not be obtained in interrogation.

Kendrick relocated lunch with their ‘guests’ to the Ritz instead, and it appears Churchill never found out.

With the arrival towards the end of the war of one of the M Room’s most difficult visitors, General Hermann-Bernhard Ramcke, described as ‘an egotistical, conceited Nazi’, ‘a firm believer in Hitler’ and ‘cold and unco-operative’, Kendrick and his team had to dig deep into their reserves of ingenuity to force him to relax and talk.

Their solution was one of sheer brilliance. Heimwarth Jestin, an American intelligence officer who was a loyal member of Kendrick’s team, recalled: ‘We drafted an artificial press release supposedly from a German newspaper.

‘It announced an award to General Ramcke from Hitler himself – not merely the coveted Order of the Iron Cross, but the very highest level of the Order, the Knight’s Cross with diamonds.

‘The citation, which we invented, mentioned the general’s bravery in defence of Brest [on the French coast]. With this false release in hand and several bottles of cognac, I visited General Ramcke.

‘Saluting smartly, I informed Ramcke of the honour Hitler had bestowed upon him and suggested we celebrate the award.

‘I produced the cognac. Despite his obvious pleasure, he declined for a moment or two, but pride in his new distinction overcame his reticence and we proceeded to celebrate in great style.’

Before long, Ramcke was talking freely, his tongue loosened by the cognac.

The Ritz Hotel in London (pictured) was where some of the captured Nazi generals were wined and dined

As he spoke, the secret listeners stood up in the M Room and cheered: he had inadvertently given away details of glider installations and troop information that he had steadfastly refused to divulge during interrogation.

Ramcke died in 1968, never knowing that his medal from the Fuhrer, which he wrote about in his memoirs, had been a fake.

In 1943 came an intelligence breakthrough of such profound significance that it without doubt changed the course of the war.

It began with comments made by a paratrooper at Latimer House, who told his cellmate: ‘I was very amused yesterday when they [the interrogation officers] showed me a drawing of the sloping ramp rocket projector.

'They know nothing about it, which is a relief to me.’

The conversation went on to reveal technical details about launch ramps – a reference to the ‘secret weapons’ which prisoners had first mentioned three years before.

Eleven days later, a bugged discussion between General von Thoma and Cruwell gave further clues to the listeners.

‘No progress whatsoever can have been made in this rocket business,’ said von Thoma.

A radio for the American Office of Strategic Services (OSS), from the Second World War. It comes with a civilian suitcase for a secret agent

‘I saw it once. The major was full of hope. He said, “Wait until next year, then the fun will start. There’s no limit to the range.” ’

The two generals had unwittingly provided, arguably, one of the most important pieces of intelligence thus far.

Earlier in the war, the British military had suspected that a site at Peenemunde, on Germany’s Baltic coast, might somehow have been involved in the production of weapons, but reconnaissance missions had failed to identify anything unusual about it.

Von Thoma’s evidence, however, was deemed to be reliable because it appeared to be an eyewitness account.

On May 26, 1943, the secret listeners overheard yet another discussion between two different officers about the V2 rocket, the world’s first long-range ballistic missile.

Captain Thomas Kendrick and wife leaving a plane at Croydon airport. Captain Thomas Kendrick, former Chief British Passport Control Officer in Vienna, was arrested by the Gestapo

It was enough to convince air intelligence chiefs.

As a direct result of these conversations at Trent Park, RAF pilots were again sent out on secret reconnaissance missions to Peenemunde.

This time, they found the evidence that had been missing before.

‘The first discoveries in the M Room about Peenemunde caused great excitement,’ recalled listener Fritz Lustig.

‘It was quickly realised that a new rocket programme was under way. Picking up this kind of intelligence for the British was very important to us because it could mean the difference between winning or losing the war.’

On June 29, 1943, Churchill authorised an attack on Peenemunde. By a full moon on the night of August 17, 1943, pilots of Bomber Command carried out the first attack in a mission that included 324 Lancasters and 218 Halifaxes.

General Hermann Bernhard Ramcke (pictured) was told that Hitler had awarded him the highest level of the Iron Cross

It came at a heavy price. More than 200 British aircrew were killed, along with hundreds of civilians in a nearby concentration camp.

But the impact of the bombing of Peenemunde cannot be over-emphasised. It bought extra time for the Allies and delayed Hitler’s first launch of a V1 on London until June 13, 1944, a week after the successful D-Day landings.

Without the intelligence from the M Room, Germany could have won the technological war, which would have made it difficult to mount the D-Day landings successfully.

The discovery was a major landmark in thwarting Germany’s race for weapons superiority.

As the Second World War approached its end, Kendrick made it compulsory for the generals to be shown footage of Germany’s concentration camps.

Their reactions were recorded in the M Room.

While some refused to believe what they saw, insisting that the film was fake, one commented that it was useless for senior officers to deny knowledge of the camps because ‘practically every German suspected that that sort of thing went on’.

Another said: ‘We are disgraced for all time, and not 1,000 years will wipe out what we’ve done.’

General Hermann Bernhard Ramcke was told that Hitler had awarded him the highest level of the Iron Cross (pictured). He never knew the citation was a British lie

Few people have ever heard of Thomas Joseph Kendrick. One reason for his obscurity is that he and the majority of his staff had to take their secrets to the grave – they had all signed the Official Secrets Act.

Even after the official files were released between 1999 and 2004, it took another 13 years for formal recognition of the M Room’s significance, after the purchase of Trent Park for luxury homes.

Key areas within the mansion are now to be made into a museum dedicated to the work of the secret listeners.

With this story emerging from the shadows, the nation can finally pay tribute to the men and women of the M Room and the commanding genius of its spy chief.

Indeed, the author and historian Leo St Clare Grondona, who led de-Nazification classes for German prisoners at Wilton Park after the war, went as far as to say: ‘Had it not been for the information obtained at these centres, it could have been London and not Hiroshima which was devastated by the first atomic bomb.’

© Helen Fry, 2019

The Walls Have Ears: The Greatest Intelligence Operation Of World War II, by Helen Fry, is published by Yale University Press on August 27, priced £18.99.