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The British Museum has reopened after a suspected noxious substance left visitors with irritated eyes and throats, as managers admitted the cause of the scare will remain a mystery.

At least 8,000 people were evacuated from the British Museum on Saturday after what was described as a "gas" incident.

Police and the fire brigade moved in but have been unable to identify the cause.

Nobody was injured during the incident.

A British Museum spokeswoman said: "The fire brigade conducted a full search but could not find anything.

"They declared the site safe and handed back control of the museum at 5pm."

She continued: "The answer won't ever be known for sure.

"It was just a strange one-off incident."

A London Fire Brigade spokesman said: "When we arrived we used our equipment to carry out very extensive checks.

"We were there for about four hours. But we did not get any readings."

The spokesman added: "People may well have smelt something but it dispersed before we got there."