The old granary building, situated six miles west of John o’ Groats, is to be converted to accommodate 10 en suite guest rooms close to the castle.

The planning application from the Castle of Mey Trust, which runs the castle and whose president is Prince Charles, has been approved by Highland Council planning officials under delegated powers.

The Queen Mother first saw the sixteenth-century Castle of Mey in 1952, when she was staying nearby with old friends Commander Clare and Lady Doris Vyner. She was still mourning her late husband, King George VI, but was immediately charmed by the building, then known as Barrogill Castle, despite half its roof having been torn off in storms. It was in a near ruinous state, but she bought and renovated it.

The granary was included in the sale of the castle, along with its walled garden and 24 acres of land.

A statement lodged from the planning application explained the background to the guest house project. It said: “The Castle of Mey Trust has been investigating diversification opportunities, in an effort to provide a broader visitor offering and facilities that will help generate higher levels of income.

“It is hoped that a range of proposals will help preserve this important heritage site for generations to come.

“The granary plays a significant role in those diversification plans. The provision of high quality visitor accommodation on site is seen as key to the success of various strands of the Castle of Mey business.

“The granary provides an obvious conversion opportunity that can be achieved sensitively and provide the level of accommodation that is required.”

The Castle of Mey was built by George, the Fourth Earl of Caithness, for his second son William Sinclair.