India's tigers are facing extinction owing to a collapse in the variety of their mating partners and the resultant lack of genetic diversity, scientists say.

Researchers at Britain's Cardiff University, working with the National Center for Biological Sciences in Bangalore, India, said they found 93 percent of the DNA variants found in tigers shot during the period of British rule in India were not present in tigers today.

The genetic diversity needed for the species to survive had been "lost dramatically," Cardiff researcher Mike Bruford told the BBC.

The researchers were given access to the Natural History Museum of London's tiger collection, which allowed them to analyze the DNA variants in tigers killed in the period from 1858 to 1947 -- the era of the "British Raj" -- but which have disappeared today.

"We found that genetic diversity has been lost dramatically compared to the Raj tigers and what diversity remains has become much more subdivided into the small (20-120 individual) populations that exist today," Bruford said.

"This is due to loss of habitat and habitat fragmentation, meaning lower population sizes, and the prevention of tigers from dispersing as they once would have, which means their gene pool is no longer mixing across the subcontinent."

Of the fewer than 2,000 tigers left worldwide, 60 percent are in India, where the territory occupied by the big cats has declined more than 50 percent during the last three generations and mating now only occurs in 7 percent of its historical territory, the researchers said.