A startling new art exhibition claims to reveal that Joachim von Ribbentrop, Adolf Hitler's foreign minister, had private plans for a stylish retirement in Cornwall following the planned German invasion of Britain.

The exhibition, and a companion book, published last week, are the culmination of three years of research by the Cornish artist Andrew Lanyon. It claims that Ribbentrop, who previously served as the Nazis' ambassador to Britain, had set his heart on a home on St Michael's Mount, one of the most picturesque locations in Britain.

In researching the Nazi diplomat's many trips to Cornwall in the 1930s for his new book and exhibition, Lanyon has uncovered a series of holiday postcards purchased by Ribbentrop that were later used to illustrate a top secret German 1942 guide to the English coastline, Militärgeographische Angaben über England Südküste.

He has also found documents and new testimony that confirm the leading Nazi's love affair with the seaside town of St Ives. They tally with local stories that the Germans had been instructed to avoid bombing particular sections of the coast.

"I have interviewed people who remember Ribbentrop's visits and the period in the 1930s when he stayed with Colonel Edward Bolitho, the Lord Lieutenant of Cornwall, in Trengwainton, his Cornish home," said Lanyon. "He told Bolitho he wanted to live in St Ives and that Hitler had agreed to give him Cornwall."

In archived documents written by Ribbentrop's aides, Lanyon has found evidence that the diplomat was frequently allowed to spend time in Cornwall because he was considered to have become a political liability in London.

"He was always exploding with rage and they clearly felt he was a real pain. But he fell in love with this area," said Lanyon, the son of the revered West Country artist Peter Lanyon, who is celebrated at the Tate St Ives gallery this week.

"At first Ribbentrop wanted to retire to Tregenna castle near St Ives. Then he was taken across to see St Michael's Mount and decided that was the right place for him."

Lanyon has also found a contemporary report in the archives of the newspaper, Cornish Evening Tidings, that claims Ribbentrop hurriedly left St Ives incognito on the train when war loomed.

"I remember my family gardener always told me that Ribbentrop wanted Cornwall," said Lanyon, "and now I believe him. My grandfather on my mother's side played golf with him and felt so dirty afterwards that he joined Bomber Command."

The artist's theory is set out in Lanyon's exhibition, Von Ribbentrop in St Ives, which opened last week at Kestle Barton. The postcards of St Ives bought by the German diplomat in affection were later used to help target the coast in the Nazi guidebook. "He turned quickly from an anglophile into an anglophobe when he realised that he had become a figure of fun in England. People were shocked when the gasworks was bombed and then people were strafed on the beaches because they had been told they were protected."

The secret dossier, which also features images of Land's End and Brighton pier, was intended to help Nazi troops find landmarks during an attack. The 1942 guide illustrates attack points along England's south coast with appealing photographs and large colour maps stretching from the tip of Cornwall to Foreness Point in Kent.

Issued by Adolf Hitler to senior officers, a rare copy was sold at auction on Thursday in Ludlow, Shropshire, by auctioneers Mullock's. The lot was listed with a guide price of between £300 and £500, but it eventually sold for £6,800 to an anonymous British collector.

Ribbentrop's connections with England pre-dated the Nazi regime's rise to power in the 1930s. As a child, he travelled to England and then returned regularly as a young adult in the wine business. He developed his love for Cornwall while based in London in the late 1930s as the Third Reich's ambassador.

In August 1934, he set up his own foreign affairs bureau linked to the Nazi party and based in Berlin. The organisation put special emphasis on Anglo-German relations; an area known to be a priority for Hitler, who held out hope of an anti-Soviet alliance with Britain.

His position as Hitler's personal envoy initially made Ribbentrop a much-courted figure in Britain and he was a frequent guest of establishment figures such as former prime minister David Lloyd George, the pacifist George Lansbury and the British fascist Sir Oswald Mosley.

Lanyon collected testimonies such as that of Michael Lyne, from Rejerrah near Newquay, who claims he met Ribbentrop in 1937 when he accompanied his journalist grandfather to an interview with the German diplomat. "We went down to a country estate and when we got there my grandfather went in," said Lyne. "Eventually grandfather came out looking blacker than thunder. Driving through Hayle my grandad said 'Ribbentrop has told me when they get world domination, he is going to live on St Michael's Mount and Cornwall will belong to him'."

In 1946 Ribbentrop was sentenced to death for war crimes at Nuremberg and was the first man executed after the trials.