For 25 years, this exquisitely enamelled medieval casket had been on loan to the Victoria and Albert Museum in London. Made in the French city of Limoges in about 1200, it was designed to hold the relics of Thomas Becket, the Archbishop of Canterbury famously murdered in Canterbury Cathedral. It had been on public display until 2000 when it was put into storage while the new Medieval and Renaissance Galleries were prepared.

Like its sister work, a slightly earlier Becket casket which was bought with the support of a public appeal 10 years ago for £4.3m, it was likely to have proved one of the centrepieces of the new galleries when they open in a few years' time.

But that looks unlikely now. Earlier this year, The Art Newspaper revealed yesterday, Metropolitan police from the art and antiques squad arrived at the V&A Museum and seized it.

So what prompted this extraordinary police action at one of Britain's most revered national museums? It followed a claim submitted last November by an aristocratic Polish family, the Czartoryskis, to the British Spoliation Advisory Panel.

The panel is an independent body set up by the Department for Culture, Media and Sport six years ago to help resolve cases involving cultural property lost - or, more accurately, stolen or seized - during the Nazi era and subsequently acquired by British museums and galleries.