The British Museum has helped to recover an important medieval Islamic artefact that surfaced in a London gallery after it was stolen from a monument in Uzbekistan.

The enormous calligraphic glazed tile – half a metre in height – had disappeared in 2014. Thieves left a gaping hole after they removed it from the magnificent entrance facade of a 12th-century monument, just over 12 miles (20km) from Bukhara, the Unesco world heritage site on the ancient Silk Road route.

Part of a high-relief turquoise glazed inscription, the tile was thought to have been lost forever until it surfaced in a Mayfair gallery, where it was being offered for sale.

The theft was not officially reported, but an Oxford scholar who had recently returned from the historic site spotted it in a catalogue published by the Simon Ray gallery.

Ray, who had bought it in good faith, immediately contacted the British Museum, which describes the tile’s recovery as “dramatic”.

The museum will this week stage an official handover to the Uzbek embassy in London.

The decorative Islamic calligraphic tile – 52.5cm (20.7in) high and 30.5cm (12in) across – had been prised off the Chashma-i Ayub monument in Vobkent. Its inscription, within a foliated scroll design, reads: “In the year five and six hundred” – which corresponds to AD1208-09.

The mausoleum at the Chashma-i Ayub complex, showing where the tile was prised from the facade. Photograph: British Museum

Dr St John Simpson, a senior curator at the British Museum, told the Guardian: “It’s a museum-quality piece.”

He said: “It is one of the finest, most beautiful, largest… earliest dated glazed tile inscriptions from a religious monument in central Asia.”

He likened the theft from such a historic site to stealing a plaque from Westminster Abbey: “It’s really quite shocking.”

He added that Ray, a leading dealer in Indian and Islamic art, had acted “honourably”.

Believed to be worth tens of thousands of pounds, the tile was identified by Prof James Allan, former keeper of eastern art at the Ashmolean Museum, Oxford. Allan and Ray contacted the British Museum immediately.

Ray said that he always does “due diligence” on any purchase, but that in some parts of the world thefts are not reported. He had been assured that the tile had been in a German collection for years.

His main concern was returning the artefact to its rightful home, he said: “I’d like to pay to put it back on the building. You can see the gap, which is quite horrifying.”

The monument is dedicated to the Islamic prophet Ayyub – known in Christianity as Job. Simpson said: “In Qur’anic tradition, Ayyub is regarded as a martyr and prophet who was rewarded with a source of water to soothe his skin afflictions, and in local central Asian folklore was viewed as a healer and a patron of silk farming, which was an important part of the economy of medieval Bukhara. The well at the centre of the monument was believed to have restorative powers.”

This tile belongs to the end of the facade inscription, which reads: “The prophet – peace be upon him – said: I had forbidden you to make pilgrimages to tombs. Now make pilgrimages. This monument was erected in the year five and six hundred.”

Simpson explained that the inscription illustrates “an important tension within Islam as to whether visiting the shrines of saints was a form of idolatry and forbidden in society”.

Abdulla Aripov, prime minister of Uzbekistan, has expressed his “sincere gratitude”. The Uzbek government is now committed to restoring the monument.

Simpson suspects that the artefact was taken from Uzbekistan on a flight to Europe and smuggled through customs.

He said: “We’re all aware of art crime and trafficking of objects in conflict zones. But what this piece shows is that we should never be complacent that monuments, even in very famous places, aren’t at risk from theft. This monument is within 20km of a world heritage site in one of the most famous countries of central Asia... It’s a stable government, a tourist destination, and yet this piece has been removed from a 10-metre high entrance. The audacity of the theft is shocking.

“That’s part of the ongoing internal investigation that the Uzbek authorities have to have. They are planning a course in Tashkent, the capital, organised with the British embassy and the National Crime Agency… to educate law enforcement officers and officials from the museum sector in detection and process in the trafficking of art – an indication of how seriously the Uzbeks are taking this.”

Dr Hartwig Fischer, director of the British Museum, said: “We see it as an essential part of our mission to help in the identification and restoration of trafficked antiquities.”

Other antiquities that the British Museum has helped to recover include an important Gandharan sculpture which was stolen from the National Museum of Afghanistan in 1996. It was returned in 2012.