Undercover guests stayed in nine upmarket hotels in New York and allegedly found some hadn't changed or washed their bedding before new customers arrived.

A shocking investigation has revealed how some hotels do not clean their sheets between guests.

Inside Edition stayed in nine upmarket hotels in New York and allegedly found three hadn't changed or washed their bedding before new customers arrived.

At a Marriott Residence Inn in Manhattan, they sprayed a message using invisible paint, that easily comes off in the laundry, on the bottom sheet of the bed.

The words which can only be seen under a UV light read: "I slept here."

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The production team checked out, leaving the bed looking as though it was slept in.

Later on they checked into the same room under a different name, and found the message was still on the bottom sheet.

"This is absolutely disgusting," said Inside Edition's Ann Mercogliano.

They allegedly found the same thing happened at the The Candlewood Inn & Suites in Times Square and La Quinta Inn & Suites near Central Park.

A manager at the Marriott was then called to the room and insisted each room's sheets were changed between guests.

Representatives for La Quinta Inn also told DailyMail.com: "At La Quinta Inns & Suites, we strive to provide a positive, consistent guest experience at all of our branded hotels.

"This includes providing a clean guestroom for every guest. We have reached out to the management team at this franchised location to understand what happened and have addressed the issue."