The mikvah dates from the 13th century and is "the only identifiable structure that has survived from the Jewish community of medieval London," according to the Museum of London.

Jews came to England at the time of William the Conqueror in 1066 and thrived here until they were expelled by Edward I in 1290. They were readmitted in the 1650s by Oliver Cromwell.

The stone mikvah was discovered in October. It consists of a semi-circular basin about four feet across and over four feet deep with a flight of stone steps leading down to it, the museum said.

John Clark, a medieval curator at the museum, called the find "very significant."

Jews were so well integrated into medieval society, Clark said, that physical evidence of their presence is hard to come by.

There is documentary proof of Jewish communities — such as deeds, contracts and receipts — but almost no archeological proof, he said. Excavated houses of Jews, for example, are identical to those of non-Jews.

"Something like a mikvah is important because it is identifiable" as Jewish, he said. "There is nothing comparable in gentile archeology."

The London mikvah is among the oldest finds of its type in Europe, Clark said. A similar ritual bath was discovered in Cologne, Germany in 1956, and is believed to date from about 1170, but there are few other examples of medieval mikvahs.

He said it was possible another mikvah had been found in London in the 1980s, but not recognized as such.

"‘A rather strange tank-like structure was found about 100 yards from where this mikvah was found. It was identified at the time as possibly a strong room, photographed, and cleared away," he remembered. "Years later, an archeologist who had worked in Israel looked at the photographs and said, ‘I think that was a mikvah.´ "

Even if it was, Clark said, it was not nearly as well preserved as the one found in October.

There is also evidence that a natural spring in Bristol, in the west of England, was used as a mikvah in the 13th century.

"Jacob´s Well," as that site is called, has the Hebrew word "zokhlin" — meaning "flowing" — carved into the rock. Clark said it is not certain that the site was used as a mikvah, but the inscription is the same age as the mikvah found in London last month.

The new find is near the site of a house that was owned in the early 1200s by "Leo Le Blund the Jew," and later by "the four sons of Abraham the Jew," records show.

The house was sold to a Christian in 1251, and the mikvah fell out of use. It was filled with dirt in the late 13th century, according to the Museum of London.

It was uncovered during construction work on the site and has now been removed.

Archeological finds of this type usually are destroyed as foundations for new buildings are laid, Clark said, but the remains of the mikvah have been treated more carefully.

The nearby Bevis Marks Synagogue — the oldest in Britain — paid to have the mikvah lifted out, and is considering rebuilding it in the synagogue's courtyard.