Some 900 people had to evacuate The Shard, western Europe's tallest building, on Thursday after smoke was reported in the London skyscraper's basement.
London Fire Brigade said there was no evidence of a blaze in the glass-clad, 310-metre (1,016-foot) tower, which pierces the British capital's skyline.
"As a precaution, the building was evacuated. It's thought around 900 people were in the building at the time," the LFB said.
The smoke dissipated and people were allowed back inside the building.
"No injuries were reported and the cause is not known at this stage," said LFB.
They said there were several possible causes for smoke without fire, such as an electrical fault.
Seven fire engines and around 50 firefighters were sent to the tower, which stands at the south end of London Bridge, but the incident was over within three hours.
Glen Cave, who was inside, told the BBC: "We had to walk to floors all the way down and it took about half an hour, but (there were) no dramas."
The Shard hosts restaurants, offices and the newly-opened five-star Shangri-La Hotel, whose guests were among those evacuated.
"We were going to be charged £590 ($990, 730 euros) a night, but now we are just standing out here in the cold now. We are finding another hotel," Wolfgang Grotzner, 62, from Hanover in northern Germany, told the Evening Standard newspaper.
Designed by top architect Renzo Piano, the 87-storey skyscraper was the tallest building in Europe when it was topped out in March 2012.
However, it was soon overtaken by Moscow's 339-metre high Mercury City Tower, which was completed in November 2012.