"The amount spent on legal lotteries remains as low as 10 per cent of the value of all underground betting," said Saowanee Thairungroj, dean of the university's Faculty of Economics, who conducted the study. "The government should find ways to terminate underground betting."

The amount spent on underground betting totalled Bt400 billion last year, while the government's lotteries drew only one-tenth of that, the survey found. The average lottery player spent Bt502.73 on government lotteries per month, while the average spent on underground lotteries was Bt1,347.9 per player.

The survey found that 33 per cent of all households in Thailand regularly placed bets on government and underground lotteries.

Saowanee said the number of lottery players was rising. The survey found residents of the Northeast, the country's poorest region, bought the most government lottery tickets, followed by Bangkok residents.

Meanwhile, those who regularly place bets on underground lotteries are typically over 50 years of age, with a monthly income of Bt5,000 to Bt10,000 each. Generally, they have only primary-school education or lower.

"Those with a bachelor's degree or higher are not fans of underground betting," Saowanee said.

A survey in 2001 found that underground betting was worth as much as Bt500 billion.

The legalisation two years ago of the underground lottery has encouraged some players to bet above ground, which has been reflected in the increasing value of government lotteries over the past two years. In 2004, the government netted Bt33 billion from the sale of lottery tickets, and it took Bt40 billion last year.