As a boy of 10, he bought a jam jar filled with coins. It became the foundation for one of the foremost coin collections in Britain, with 1,000 of Scotland’s oldest coins worth a total of more than £500,000.

But just as Lord Stewartby, 71, a former government minister, was about to start cataloguing his treasures in retirement, the collection has been stolen from his country home, Broughton Green, which featured in the classic thriller The 39 Steps.

Among the items lost was a coin struck during the reign of Robert the Bruce more than 650 years ago. Others were minted under his son, David II, in the 14th century, under James I in the 15th and his son, James II, also in the 15th century.

Lord Stewartby’s daughter, Lydia Pretzlik, 38, spoke yesterday of her father’s shock at the theft. She said: “One of my father’s frustrations is that it is the whole collection that is so important, not the commercial value. It’s what it represents in historical terms.”

As Sir Ian Stewart, her father was Economic Secretary to the Treasury in 1983-87 with responsibility for the Royal Mint, overseeing the issue of the first £2 coin to celebrate the Commonwealth Games in Edinburgh.

After a long career in the City and politics he retired in May as deputy chairman of Standard Chartered bank and he and his wife went on holiday. While they were away last month, thieves broke into their house in the village of Broughton, near Biggar in the Scottish Borders.

Mrs Pretzlik said: “When he was 18 he wrote the seminal book on coins, The Scottish Coinage, and he was hoping to update it. It is his life’s work in many ways and this is a big shock to him. It would be a terrible shame to see such a collection lost for ever.

“This is a matchless archive and we are all clearly very anxious about what will happen to them, but my father is hopeful that they will be returned.”

Lord Stewartby has offered a reward, believed to be a six-figure sum, for the safe return of the collection.

Little else is believed to have been stolen from the house, where Lady Stewartby’s grandfather, the author John Buchan, lived. It featured in several of his books, including his best-seller, The 39 Steps.

Police have not ruled out the theory that the Scottish coins were stolen to order. A spokesman said: “We have carried out a significant amount of inquiry so far and are now appealing to the public for help. We would appeal to anyone who may have information as to the whereabouts of the collection or anyone who has been offered some rare and unique coins.”

Nick Holmes, curator of numismatics at National Museums Scotland, described the loss as “a total disaster” for the country.

He said: “Lord Stewartby has been putting this collection together over decades. One of his main reasons for doing so was so it could be used as an academic resource. The collection is unique and important both for its quality and quantity.”

Sir Ian Stewart was Conservative MP for Hitchin, 1974-83, and for North Hertfordshire, 1983-92. He took his seat in the Upper House as Lord Stewartby in 1992. He had previously served as a Treasury, Defence and Northern Ireland minister .

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