A small walrus tusk warrior figure bought for £5 in 1964 – which, for years, was stored in a household drawer – has been revealed as a missing piece from one of the true wonders of the medieval world with a possible value of £1m.

The Lewis chessmen hoard was found in 1831 in the Outer Hebrides and the elaborately carved pieces soon became stars of museum collections in London and Edinburgh. They have also become well-known in popular culture from Noggin the Nog to Ron Weasley’s perilous chess game in Harry Potter and the Philosopher’s Stone.

The original hoard comprised 93 objects with the whereabouts of five pieces always a mystery. Until now. On Monday, the auction house Sotheby’s announced it had authenticated a missing piece and would sell it in July with an estimated value of between £600,000 and £1m.

The missing piece, measuring 8.8cm in height, is a Lewis warder and – 55 years ago – was purchased for £5, about £100 in today’s money.

It was originally labelled “antique walrus tusk warrior chessman” and bought in Edinburgh by an antiques dealer who passed it down through his family, who have asked to remain anonymous.

A family member said it had been stored away in their grandfather’s house, with everyone unaware of its importance. “When my grandfather died my mother inherited the chess piece. My mother was very fond of the chessman as she admired its intricacy and quirkiness.

“She believed that it was special and thought perhaps it could even have had some magical significance. For many years it resided in a drawer in her home where it had been carefully wrapped in a small bag. From time to time, she would remove the chess piece from the drawer in order to appreciate its uniqueness.”

Alexander Kader, the Sotheby’s expert who first examined the piece for the family, said his jaw had dropped when he saw it, and he knew straight away what it was. “I said: ‘Oh my goodness, it’s one of the Lewis chessmen.’”

He added: “They brought it in for assessment. That happens every day. Our doors are open for free valuations. We get called down to the counter and have no idea what we are going to see. More often than not, it’s not worth very much.”

The Lewis chessmen were most likely made in Trondheim, Norway, of which the Western Isles were part, between 1150 and 1200.

Lewis was on a thriving trade route between Norway and Ireland and one theory is that they were hidden for safekeeping by a travelling merchant.

They became arguably Scotland’s best known archaeological find when they were found buried in the beach of Uig Bay in 1831. How they were discovered is still disputed, with one account claiming they were uncovered by a grazing cow.

The chessmen fell into the hands of an Edinburgh dealer who sold 10 pieces to the Society of Antiquaries of Scotland, which could not afford a greater number, and 82 to the British Museum. As well as the chess pieces, the hoard includes 14 “tablemen” gaming pieces and a buckle.

The fact that London has the majority of the chessmen objects, 82 against 11 now in Edinburgh, has often been a sore point, with the Scottish National party arguing for the chessmen to be permanently returned to Scotland.

The former British Museum director Neil MacGregor included them in his A History of the World in 100 Objects series for BBC Radio 4, and talked of how the “much-loved pieces take us into the heart of the medieval world”. He said they showed not only high status, wealth and power, but also “knowledge, taste and intellect”.

The chessmen are made mostly from walrus tusk, a few from whales’ teeth, and consist of seated kings and queens, bishops, knights on horseback, pawns in the shape of obelisks and standing warders. Some of them would originally have been coloured red.

The warders, equivalent to rooks on a modern board, are the most intimidating of the chess pieces, some of them biting on the edge of their shields in an ecstasy of rage which identifies them as berserkers, the shock troops of the Scandinavian medieval world.

Kader, head of European sculpture and works of art at Sotheby’s, said the discovery was “one of the most exciting and personal rediscoveries to have been made during my career”.

He added: “Today all the chessmen are a pale ivory colour, but the new Lewis warder’s dark tone clearly has the potential to offer valuable and fresh insight into how other Lewis chessmen may have looked in the past.”

• This article was amended on 10 August 2020 to correct the spelling of the word “berserker”.