Two big cats were caught on camera by rangers who were filming a deer survey, the Forestry Commission revealed today.
Under a Freedom of Information Act request, the government agency confirmed that two "reliable" sightings of large cats have taken place in the last seven years.
Experts monitoring deer using thermal imaging cameras spotted the animals on two separate occasions in different parts of the Forest of Dean, Gloucestershire.
Commission spokesman Stuart Burgess said the sights had been confirmed by "very experienced" rangers unlikely to mistake deer for big cats.
He said today: "Both were observed in low light, using heat-activated vision equipment while they were carrying out a deer census. The colour of the animals couldn't be made out, but these are very experienced guys and they know what is and what isn't a deer. One definitely believed that what he saw was some sort of large cat."
The first sighting was at the outskirts of Churchill enclosure, east of Parkend in February 2002. The second was on the southern slopes of Staple Edge in March 2005.
There was no sign of large cats during the most recent deer census in March 2008, however.
Gloucestershire police also suspect there are pairs living in the Forest of Dean and around Cirencester and say they "take sightings seriously".
The force's wildlife and environmental crime officer, Mark Robson, who gathers evidence about glimpses of the creatures, said today: "I have been looking at big cat sightings since 2002 and believe the two examples to be credible sightings.
"Gloucestershire Constabulary gets around 100 sightings a year from members of the public. Most of these are at some distance, it's very rare to have a close encounter.
"If you suspect you've seen a big cat you should jot down the details and contact the police. We will take sightings seriously and will investigate."
The sighting is one of several in the Forest this decade.
In November 2006, schoolboys Joe Tingle and Chay Maidment ran in terror after a big "green-eyed" animal came towards them through undergrowth near Ruspidge.
In March, a big cat was seen by a dog walker who believed he had met a leopard.
A survey by the British Big Cats Society showed more than 2,000 suspected big cat sightings were reported nationwide between 2004 and 2005.
Besides Gloucestershire, only Devon and Yorkshire recorded more sightings among the English regions during that time.
Most wild cat sightings are attributed to panthers, with a handful of reports put down to their smaller cousins the lynx, once native in this country.
Their freedom in the wild is attributed to a range of sources from escaped zoo and circus animals to a mass release in the 1970s after the enforcement of the Dangerous Wild Animals Act.
Deputy surveyor Rob Guest, who was present at both sightings, said today that his team were "interested, excited, and surprised" when the unmistakable thermal images appeared in the gloom.
It was the clear shape of the animals' long tails that told Mr Guest's crew they were not dealing with a deer, he said.
He said: "I was driving at the time, which is quite something in itself through the forest at night. The images showed up white on the thermal imaging equipment which registers body heat.
"You basically get a white image upon a black background. It was clear enough so that the tail was very, very obvious. It was more than a deer.
"I couldn't tell you how big they were or what they were. They were just large, full cats.
"At that time, going back five years, there were lots of sightings and rumours. At the second sighting in 2005, there were a lot of rabbits living in that particular area, and studies have shown that, when cats go wild, rabbits are a major source of their food."
One theory was that people had kept them and then released them after stricter laws came into effect, he said.
Mr Guest, 58, who has worked for the Forestry Commission for 18 years, said these were the only two wild big cats he had seen in his career.
He continued: "It may just have been one or two ranging over huge distances. The forest is fairly wild country anyway, so that may be the reason they were there.
"We weren't scared, but more interested, excited and surprised."
The deer surveys, which take place every three years, are carried out all night for three or four weeks.
At the time of the sightings Mr Guest was accompanied by only one or two colleagues.