A British brigadier’s teenage daughter trapped in France by the Nazi invasion was suspected of becoming a Gestapo officer’s mistress, previously-secret MI5 records reveal.

Antonia Lyon-Smith was just 15 and separated from her parents when the Germans arrested her and sent her to an internment camp.

She was released on the grounds of her age, but ran into trouble again after becoming involved with a prominent Resistance group and was threatened with being deported to a concentration camp in Poland.

But a Gestapo chief saved her by taking her on as a housekeeper in his Paris offices, where a Nazi officer called Karl Gagel fell in love with her and made her promise to marry him once the war was over.

Ms Lyon-Smith was interrogated by MI5 but disclosed nothing about her relationship with Mr Gagel, leading a British intelligence officer to conclude that she was his mistress and “almost certainly” betrayed all her know-ledge of the Resistance cell to the Germans.

In her own memoirs, published in 1982, she revealed that Mr Gagel was only one of four men she became engaged to during the war – none of whom she went on to marry.

The daughter of Royal Artillery officer Brigadier Tristram Lyon-Smith and a Canadian mother, the English schoolgirl was staying with her cousin in Concarneau, Brittany, northern France, when World War II broke out.

In December 1940 German soldiers took her into custody, along with other Britons, and held her in an internment camp in Besançon, eastern France.

She applied for release on the grounds that detainees had to be over 16. After being freed in March 1941, she went to Paris by herself and resumed her schooling.

In the summer of 1942, Ms Lyon-Smith decided to try to escape from France, but her attempts to flee over the Swiss border or make it to Spain with the help of the American Embassy failed.

She returned to Paris in September 1943 and was asked by a man called Claude Spaak, the brother of the Belgian foreign minister, to write a letter of introduction to help a friend of his gain safe passage into neutral Switzerland.

The British schoolgirl later insisted to MI5 that she did not realise at this time that Spaak was heavily involved in the Resistance.

It appears from the files that she was involved – perhaps unwittingly – in attempts to assist the escape of Leopold Trepper, the Soviet Union’s main spy in France.

But the Nazis intercepted Ms Lyon-Smith’s letter. She was arrested by senior Gestapo officer Heinz Pannwitz and taken to his offices in Paris for interrogation.

She claimed she was held there in solitary confinement until January 1944, when she was allowed to leave and stay with her cousin, who had by this time moved to Paris.

But her MI5 file, newly released by the National Archives, includes a report from a Nazi source suggesting that she “did little but make tea, sew and listen to radio” and had a “fair amount of freedom” while with the Gestapo.

She was even “frequently accompanied on shopping expeditions in Paris” by Mr Gagel, it notes.

MI5 became involved when Mr Gagel tried to contact her after the war via her father’s branch of Barclays Bank in Hove, East Sussex.

The former Gestapo officer wrote to the bank in October 1945: “I should be much obliged if you would kindly inform Ms Antonia Lyon-Smith that I shall be in Germany for some time to come, and that I should like to have news of her.”

Brigadier Lyon-Smith was furious about this letter and asked British intelligence to track down Mr Gagel.

An MI5 officer who interviewed the army officer about his daughter noted: “She was apparently ‘befriended’ by Karl Gagel, who ostensibly arranged that she should not be sent to Fresnes prison in return for her undertaking to marry him when the war was over...

“Antonia has not the slightest intention of ever seeing Karl again if she can possibly avoid it.”

But Ms Lyon-Smith was much less forthcoming when she was questioned by MI5 officer Major John Gwyer in March 1946.

The file records that she provided a “rather disconnected” account of her time in France and was not at all helpful when asked to describe the Gestapo officers who held her in Paris.

“After a considerable lapse of time and much wandering around the point, she finally also contributed the information that Mr Gagel might have come from Worms and was in civil life connected with some trade or business,” Maj. Gwyer recorded.

“Taken as a whole, the interview was not satisfactory. I cannot say that Lyon-Smith appeared evasive or obstructive, but at the same time she succeeded in conveying remarkably little inform-ation and was liable to constant lapses of memory when invited to give addresses or other concrete facts.”

The officer concluded: “I gave her ample opportunity to tell me what she knew of Mr Gagel. It is clear to me that she was holding back on this matter, though whether simply because her association with him was a disreputable one or not, I cannot say.”

Mr Gagel was eventually tracked down and interrogated in December 1949, when he claimed that he and Miss Lyon-Smith had an understanding that “when Germany and Britain ceased to be enemies they would become engaged”.

He admitted that he had tried to contact her two or three times since the war.

His interrogation report notes: “(Gagel) himself, against his will, believes that Antonia gave his letters to the British authorities, but nevertheless is still anxious to get in touch with her, if only because she is in possession of a considerable amount of money and a valuable camera which he left in her custody.”

Maj. Gwyer raised concerns in June 1946 about whether Miss Lyon-Smith was suitable to continue working for the Women’s Royal Naval Service.

But Royal Navy intelligence wrote back saying that the problem had been already resolved because she had become engaged and had applied to leave the service after her wedding.

Her file includes a notice from The Times recording her marriage to Royal Navy Lieutenant David Ellis near Bridport, Dorset, on June 20, 1946.

Ms Lyon-Smith’s book about her wartime experiences, titled Little Resistance and published under her name from her second marriage, Antonia Hunt, confirms that she kissed Gagel but she insisted they did not sleep together.