For a century it had been a treasured heirloom, passed down through generations, peeked at through the glass of a gloomy medals case.

Gen Sir Harry Prendergast’s Victoria Cross – the highest and most prestigious award for gallantry in the face of the enemy – was, for his family, a symbol of his astonishing bravery during the Indian Rebellion of 1857.

But now, 55 years after it was loaned to the National Army Museum (NAM), his descendants fear they will never see the original again.

In the years after its loan in 1962, the VC was, his family believe, stolen and replaced with a copy – something they say two of the UK’s most respected institutions are failing to acknowledge.

It’s a mystery Kay Gladstone, Sir Harry’s great grandson, and his family has been trying to solve since 1983, when William Reid, then director of NAM, first wrote to tell them an expert at Hancocks (who hold the monopoly on producing VCs) realised it was a copy.