Documents showing sensitive customer details have been found in a bin at the rear of the Oasis in Swindon.
The discovery was made by members of the Save Oasis Swindon campaign, who are fighting to prevent the iconic leisure centre from being permanently closed.
They found the documents while checking whether rubbish fly-tipped there previously had been removed.
GLL, the company which ran the Oasis until handing the lease back to leaseholder Seven Capital, has pledged an investigation.
One the Save Oasis Swindon campaigners, who asked not to be named, said: "I found the data in a normal waste bin behind the Oasis.
"There were customer names, staff names, kids' names, payment schedules, membership details, swim class details, staff diaries, printouts of recent emails, timesheets.
"Some of the data concerned customers for not only the Oasis, but the Link Centre, Health Hydro and Delta Tennis Courts. This was only the stuff at the top of the bin.
"None of the data had been shredded or disposed of appropriately. We did not see any bank information, so we don't want to worry people too much. Mainly names and payment dates."
The campaigner added: "We were pretty horrified by this find. We immediately took all the necessary steps to ensure this data was safe.
"We reported our find immediately to the head of Swindon Borough Council and also to the Information Commissioner.
"We are quite shocked as this has obviously been left there for three months since its closure. GLL were obviously in a rush to get out of the Oasis so this was obviously hastily disposed of."
A GLL spokesperson said: “We are aware of the allegations made over the weekend on the Save the Oasis Facebook page regarding data security at the Oasis Leisure Centre.
"We will work with Swindon Borough Council and the Information Commissioner’s Office to ensure that there is a full investigation and that any personal data is protected.”