LONDON — The British Museum plunged itself into a geopolitical tempest on Friday, lending one of the most disputed Greek artifacts to Russia’s State Hermitage Museum in a surprise arrangement that outraged Greece.

The loan of the artifact, one of the so-called Elgin marbles, a British Museum collection of Parthenon sculptures, also seemed at odds with the West’s increasing ostracism of Russia over the Ukraine conflict and a range of other East-West disputes.

Not only was it the first time that the British Museum has lent part of the collection, which Greece contends was looted by Lord Elgin in the 19th century, but it also was done in secret.

The British Museum announced the loan only after the artifact, a headless statue of the Greek river god Ilissos, was spirited to the Hermitage in St. Petersburg. It will be on display from Saturday through Jan. 18 and will form part of an exhibition celebrating the museum’s 250th anniversary.