A group of Tottenham players were exposed to a virus before arriving at a hotel where they ate a meal later blamed for their illness.
Ten Spurs players were taken sick ahead of their crucial final-day Premiership game against West Ham, which they lost 2-1.
Their request to replay the game was later turned down by the Premier League.
Tests this week cleared the Marriott Hotel in Canary Wharf, London of food poisoning. A batch of lasagne was mistakenly blamed for the illness.
An investigation by the Health Protection Agency and Tower Hamlets Environmental Health has now established they must have been infected beforehand.
In a joint statement they said: "Results have confirmed that Norovirus, a form of viral gastroenteritis, has been identified in three samples taken from those affected.
"Although no conclusion can be drawn as to the source of the illness, as the symptoms of Norovirus infection begin around 12 to 48 hours after a person becomes infected, exposure to the virus will have taken place prior to their arrival at the hotel."
Dr Alex Mellanby, consultant in communicable disease control at the North East and Central London Health Protection, said: "Our investigation has identified Norovirus as the cause of the outbreak.
"The affected cases would have been exposed to the virus before they visited the hotel and the infection was probably transmitted from person to person."
The statement added: "Whilst the investigations by the Health Protection Agency and Tower Hamlets Environmental Health have identified the cause of the outbreak, both organisations will continue to provide advice to the football club in any further investigations they undertake." PA