More than 50 people have now attended hospitals after suffering suspected carbon monoxide poisoning at a go-karting track.
Lincolnshire Police and public health officials launched an investigation after a number of people complained of symptoms after visiting Gridline Racing Indoor Karting in Lincoln on Tuesday night, 15 August
The Lincolnshire Integrated Care Board initially said 33 people were suspected to have become unwell.
But on Thursday a spokesperson said others had since been to hospitals run by the United Lincolnshire Hospitals NHS Trust.
The spokesperson said: "As of this morning a total of 56 patients attended ULHT sites with symptoms of carbon monoxide poisoning in relation to this incident. One patient remains in hospital."
Gridline has apologised following the incident and confirmed it remained closed.
In a statement it said: "We are extremely sorry to all those affected. All of us at Gridline are incredibly upset by the news and would like to assure you all that we will update again once we understand the situation and the investigation is complete.
"As this is an ongoing incident and a full investigation is ongoing, where we continue to work with Lincolnshire Police and other relevant organisations, we are unable to comment further at this time."
Lincolnshire police said they had contacted anyone who visited the track on Tuesday.
Health officials said they were working with "multiple agencies" to support anyone affected by the incident.
Gridline's statement added: "We are asking anyone who has not received contact from Lincolnshire Police and is experiencing headaches, dizziness and nausea following their visit to call or visit NHS111 to seek appropriate medical advice."
Anyone who visited the track and is experiencing headaches, dizziness and nausea is urged to seek medical advice.
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