An inquiry into how 10 Tottenham Hotspur players fell ill at a hotel just before a game has begun.
Manager Martin Jol said he wanted the game to be postponed
The players apparently suffered food poisoning before the Premiership match.
Spurs asked for Sunday's fixture at London rivals West Ham - vital for Champions League qualification - to be put back a day, but officials declined.
Food samples from the Marriott Hotel in east London are being tested by health authorities. An official criminal investigation has not been launched.
'Testing under way'
In a joint statement, the Health Protection Agency and Tower Hamlets Environmental Health confirmed they were now investigating a "possible outbreak of food poisoning" and working with the football club and the hotel to identify any possible sources.
Colin Perrins, head of Tower Hamlets Trading Standards and Environmental Health, said: "Samples have been taken and are in the process of being analysed at independent government laboratories."
Robbie Keane (played 90 mins)
Edgar Davids (90mins)
Michael Dawson (90 mins)
Aaron Lennon (90 mins)
Michael Carrick (63 mins)
Teemu Tainio (87 mins)
Radek Cerny (unused sub)
Tom Huddlestone (did not play)
Lee Barnard (12 mins)
Calum Davenport (3 mins)
Test results are expected on Thursday at the earliest.
Paul Downing, general manager of the London Marriott West India Quay Hotel, in Canary Wharf, said it was not yet clear what had caused the outbreak.
Although it is not the subject of a criminal investigation, a Metropolitan Police spokesman said the food samples were taken from the hotel "as a precaution" and that there was "no official allegation".
Tottenham manager Martin Jol said he did not suspect foul play in the incident which saw first team players including Michael Dawson, Edgar Davids, Michael Carrick and Robbie Keane struck down with the illness.
But all four played in the game on Sunday which Spurs lost 2-1. The result helped north London rivals Arsenal to secure fourth place and a Champions League spot ahead of them.
Spurs informed the Premier League on Sunday morning they were in danger of being unable to fulfil the match, and Jol asked for the game to be postponed until Monday or at least until 1800 BST on Sunday.
But Premier League chairman Richard Scudamore said he was not able to deem the match a "postponeable event" because Tottenham still had enough fit players to fulfil the fixture.
After consultation with the police the best offer from the Premier League was to delay kick-off until 1700 BST.
Mindful of a decision in 1997 to dock three points from Middlesbrough after they failed to fulfil a league fixture, Spurs decided to go ahead with the game as scheduled.
Bernie Kingsley, editor of the Spurs fanzine, told BBC News: "Tottenham fans at this stage blame the Premier League for not being bolder and taking the decision to postpone the game - which is what they should have done.
"They should have taken it out of the club's hands, they should have said the club does not have enough fit players, it should have been played 24 hours later, and that would have kept everyone happy."