London's Millennium Bridge "wobbled" so much when it opened because of the way people walked across it, a study by civil engineers has claimed.
The £18m Thames crossing was famously shut after two days in 2000 as police were worried it swayed so much.
This was because people naturally move from side to side when walking rather than in a truly straight line, research at the University of Bristol suggested.
The same problem arose on other bridges tested as part of the study.
The 320m (1,067ft) span, which links St Paul's Cathedral on the north bank of the river to Tate Modern in the south, was the first new Thames crossing in more than a century.
It was designed by the architect Sir Norman Foster with sculptor Sir Anthony Caro.
About 100,000 people used it on its inaugural weekend but many found it swayed alarmingly in the wind, leading engineers to conduct safety checks.