The museum is home to the Elgin Marbles At least 3,500 people were removed from the British Museum in what was described as a “gas” incident.
Visitors to the museum in Bloomsbury, central London, complained of an odd smell and irritation in their eyes and throats.
A Metropolitan Police spokesman said: “Purely as a precaution everybody was evacuated from the building.
“We think it is something that got into the air conditioning system.”
No one needed hospital treatment.
A British Museum spokeswoman said: “There was a gas incident in the museum at about 12.30.
“There were no injuries but the museum was promptly evacuated. Police are investigating to ascertain what happened.”
The London Fire Brigade was also at the scene.
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