The PPP, whose election victory last month left it eight seats short of winning a majority, has been deterred since then by legal and other hurdles from setting up a coalition government.
In the latest ruling, the high court dismissed a complaint that said the PPP is a proxy of the defunct party of Thaksin, who was deposed in a military coup in 2006.
The court said it had no authority and that the case must be settled by the Constitution Tribunal, the Bangkok Post reported. An adverse ruling could have invalidated the Dec. 23 election outcome and the PPP dissolved, the report said.
The court also dismissed all other lawsuits challenging the Dec 23 elections. The new government may be in place by next week, it said.
The rulings would make it easier for Thaksin to return to the country from exile to face corruption charges, which he has denied. His lawyers have said he would return only after an elected government is in place so he can get a fair trial.
The PPP is likely to install its leader Samak Sundaravej as the next prime minister.