"The counting will be done in two phases this time. The first phase which began on Thursday will continue till January 10," said Atanu Raha, chief conservator of the Sunderbans forest in the eastern state of West Bengal.

"In the first phase, wildlife officials will take photographs of the tigers, count their numbers and put radio collars on four tigers," Raha said. The four tigers to be fitted with collars will be tranquillised and then released again.

"The radio collars will be linked to a satellite through transmitters. We will be able to read their movements. "The second phase will begin by the end of January when experts will make an intensive survey through collecting prints of the paws," Raha said.

The census is part of an initiative to preserve the bio-diversity of the mangroves, the world's largest habitat of the cats and a World Heritage site, 100 kilometres (62 miles) south of Kolkata, capital of West Bengal.

Bengal tigers are nearing extinction because of rampant poaching for their coveted skins and bones as well as habitat loss due to human encroachment. A single Bengal tiger can fetch up to 50,000 dollars.

At the last count in 2004, there were 274 tigers in the Indian part of the marshlands straddling India and Bangladesh. Zoologists estimate that Bangladesh has nearly 325 tigers.

Forest official Raha said that the new computer programme would reduce chances of committing errors in the census - either by missing out some cats or counting others twice as they cross over from Bangladesh.

The Sunderbans cover 10,000 square kilometers (3,860 square miles) of mangrove marshlands off the coast of the Bay of Bengal. Nearly one-third of the area is on the Indian side.

According to government estimates, between 3,500 and 3,700 tigers in all are left in the wild in India, down from 40,000 tigers before India's 1947 independence from Britain.

Experts believe there were around 100,000 tigers in Asia at the start of the 20th century.