Health officials investigating a suspected case of Legionnaires' disease at Terminal 5 have found the bacteria on the site.
A Laing O'Rourke worker was taken to hospital two weeks ago with suspected Legionnaires' disease, which has now been confirmed.
Health tests this week revealed the presence of Legionella bacteria in a water tank used in concrete production. The Health and Safety Executive is investigating.
A BAA spokesperson said: "Interim tests from an isolated tank used to supply hot water to a concrete batcher have now revealed the presence of the bacteria.
"It is not confirmed whether this is the source of the worker's illness but every precaution has been taken to ensure there is no risk to anyone on the site."
The tank has now been taken out of operation. The employee, who has not been named, remains in a serious condition. No other workers have as yet exhibited symptoms.
Meanwhile, contractor Laing O'Rourke has bowed to civil engineering workers' demands for pay increases in a bid to stave off industrial action.
On Monday union officials agreed to recommend that workers accept Laing O'Rourke's offer of a bonus pay increase of 67p an hour, 33p short of the claim for an extra £1 an hour.
However, the offer is dependent on a cancellation of a four-day strike scheduled to start on Friday. As Building went to press on Wednesday workers had not voted.
A Laing O'Rourke spokesperson said: "This one-off improvement to bonus arrangements, which is to last the duration of the T5 project, confirms that workers at T5 are some of the most productive and enjoy the best terms in the industry.
"We would expect union members to accept the recommendation of the national secretaries and agree to the new bonus offer."
This dispute is one of several to hit the project. It emerged this week that Amicus, which represents some production workers, has called on BAA to eject M&E subcontractor Sepam from the project because of a row over employment practices.
Amicus claims Sepam has flouted laws on redundancies and pay. It says the company must sign up to the JIB national agreement by the end of the week.
A BAA spokesperson said it was working with Sepam to resolve the issue. She said: "We are working closely with the supplier to ensure it meets all of T5's requirements."