The recently announced $1.87bn sale of Thailand's largest mobile phone operator, Shin Corporation, could be against the law, and poses a security threat to the country, economic researchers said this week.

Opposition politicians have already questioned the propriety of the tax-exempt deal, in which the main beneficiaries are members of the Thai prime minister's immediate family.

The sale could leave Temasek Holdings, a Singaporean group, with indirect control of more than 75 per cent of Shin's shares, contrary to Thai laws which limit foreign stakes in local businesses, the Thailand Development Research Institute said. Temasek is wholly-owned by Singapore's government.

Thai prime minister Thaksin Shinawatra founded Shin Corporation in 1987, and passed his shares in the company to family members when he took office in 2001.

The company controls Thailand's largest mobile phone operator, Advance Info Service, and has substantial stakes in a TV station, a satellite communications company, and an airline.

Shinawatra has accused critics of the $1.87bn sale of "jealousy", according to media reports.

In related news, Thai intelligence officials are concerned that Singaporean authorities could use the satellites to eavesdrop on Thai communications, Thailand's defence minister told local newspaper The Nation.