Security chief Thawil was reinstated to his post in March but the Constitutional Court accepted a case brought against Yingluck by 27 senators who argued his removal had violated the constitution.

"The prime minister will be given until May 2 to present her defence and gather further evidence," Somrit Chaiwong, a Constitutional Court spokesman, said.

The court will then hear witnesses on May 6 and its judgment could come any time after that.

A verdict that removes Yingluck will escalate tension between her supporters and the anti-government protesters. Some fear an increase in violence that could prompt intervention by the coup-prone military.

A long-running crisis broadly pits the mostly poorer supporters of Yingluck and her brother, former premier Thaksin Shinawatra, against the Bangkok-based middle class and the conservative establishment. Thaksin was ousted by the army in 2006 and has chosen exile rather than accept a jail term handed down in 2008 for abuse of power.

The protesters want to end Thaksin's political influence and set in motion broad political reforms that would prevent parties loyal to him from running for office again.

A Feb 2 election was disrupted by the protesters, who stopped candidates from registering and blocked polling stations. As a result, a court ruled the election void and the Election Commission has said the earliest date for a rerun is July 20, but even that looks optimistic.

Despite occupying key government ministries for weeks at a time and bringing parts of the capital, Bangkok, to a halt until retreating to a city park in March, the protesters have failed to achieve their aim of bringing down the government.

They are now looking to the Constitutional Court and the National Anti-Corruption Commission (NACC) to remove Yingluck. Her supporters believe the courts and other state bodies are biased against her and Thaksin.

Amongst the charges she faces is one of dereliction of duty. She is the nominal head of a state rice-buying scheme that critics say is riddled with corruption and has run up huge losses.

The NACC, which brought that charge against her, is expected to deliver its ruling in May. If found guilty, she could be removed from office and may get a five-year ban from politics.