A Thai court has refused to open an inquiry into claims Prime Minister Thaksin Shinawatra broke conflict of interest rules.
Mr Thaksin has been dogged by protests over the share sale
A group of 28 senators had alleged Mr Thaksin kept control of his family's telecoms giant, Shin Corp, after he sold his shares to his family in 2001.
The court ruled eight to six against an inquiry, citing inadequate evidence.
Mr Thaksin has faced calls to resign since his family sold its stake in the company last month.
The sale, which netted the family members and others $1.9bn, has angered many urban Thais, who complained the family avoided paying tax and passed control of an important national asset to Singaporean investors.
One of the judges, Jumpol na Songkhla, told Reuters news agency that the allegations - that Mr Thaksin influenced government policy to help the company he built - were not detailed enough.
He said the senators who made the complaint would have to make more specific allegations if they wanted the court to get involved.
Mr Thaksin's critics hoped the Constitutional Court's involvement would begin a process that could have eventually led to Mr Thaksin's being ousted from power.
Analysts said the court's decision should ease the pressure on Mr Thaksin, but was unlikely to end it.
His opponents have announced they will hold the latest in a string of demonstrations against him on 26 February.
Mr Thaksin, who was returned to office last year in a landslide, still has a large parliamentary majority, and his rural support base has shown no signs of wavering.