HotelHippo.com, owned by HotelStayUK, had revealed booking information that had been a "gift for burglars", a security expert said.
The exposed data could allow the matching of hotel bookings with home addresses.
After being contacted by the BBC, HotelHippo.com was taken offline.
In a statement, the company said: "We confirm that we have taken down the HotelHippo.com website to take some urgent action to deal with a technical situation.
"Privacy of customer data is our prime concern, and we are committed to ensuring this safety."
Information security consultant Scott Helme said he had sent details of the vulnerability to the firm on 25 June, but no action was taken until Tuesday.
HotelHippo, based in St Albans, offered bookings with large chains including Marriott Hotels and Radisson. Other sites owned by HotelStayUK offer theatre tickets and other tourist experiences.
Mr Helme, who described the breach as "appalling", told the BBC that repeated emails and phone calls to HotelStayUK had been ignored.
However, managing director Chris Orrell said he was unaware of the issue.
"No-one's passed on any information to me," he said.
Address database
The UK's data privacy watchdog, the Information Commissioner's Office (ICO), opened an investigation on Tuesday.
"We will be looking into the matter to establish the full details," a spokesman said.
Despite the website displaying several messages and trust stamps stating it was "secure", Mr Helme said he had discovered the vulnerability with ease.
"I easily discovered a method of extracting the personal and sensitive data of thousands of customers that had used the site before me," he said.
The vulnerability centred on the use of unique web addresses to pull up customer data.
When placing a booking, a unique five-figure number would appear in the address bar of the web browser.
By simply altering this number, any user could pull up details of previous bookings.
The leaked data included the date, location and length of a hotel stay. On a separate page, the home address of the person who made the booking could also be found.
Mr Helme said a simple program could be written to pull the data from the site - essentially creating a database of addresses where the residents were staying at hotels, and for how long.
Source: BBC News
