Thermal pictures taken by The Telegraph show 90 percent of Occupy London tents were empty overnight. Source: The Telegraph

An analysis of images taken by a thermal camera has found that 90 percent of Occupy London tents are empty overnight, a claim the group disputed today.

The Telegraph newspaper took the images of the occupied space outside St. Paul’s Cathedral Monday at 11 p.m.

Red and yellow spots in tents indicate body heat and that a person is inside, while cooler colors, such as blue and purple, mean no one is inside the tents.

Only 20 of the camp’s 200 tents appeared to have someone inside, the newspaper reported.

Naomi Colvin, an Occupy London spokesperson, called the use of heat sensing cameras an “invasion of privacy,” and said the analysis did a poor job of estimating the number of people inside the camp.

“We had at least 140 people that night,” Colvin told ABCNews.com,  noting that the group uses sign in and sign out sheets to keep track of who is inside the tent city at any given time.

Colvin contended that the encampment is consistently two-thirds full and that people share their tents with others if they know they will be gone for a night.

“These are real  people with real lives and responsibilities,” Colvin said. “It’s impossible to expect someone to stay every single night, but the fact that we are here shows we are deadly serious.”

The group has been occupying the area outside St. Paul’s Cathedral since Oct. 15. The famous landmark was forced to close its doors last Friday for the first time  since World War II due to what what it called “health and safety concerns.”

“I am asking the protesters to recognize the huge issues facing us at this time and asking them to leave the vicinity of the building so that the cathedral can re-open as soon as possible,” Graeme Knowles, dean of St. Paul’s, wrote in an open letter posted on the cathedral’s website.

The landmark is losing an estimated $25,000 per day in ticket sales. Many protesters have vowed to stay until the summer Olympics, which will be held in London next year.

The City of London Corporation said on its website it will hold a closed meeting this Friday to discuss possible legal avenues for evicting the protesters.