The former army chief has been in military custody since February 8, when military police arrested him on charges of engaging in politics against Mahinda Rajapaksa, Sri Lanka's president, while still in uniform.
'Rigging the vote'
His arrest came after he lost by 1.8 million votes in a January 26 presidential election, after which he accused Rajapaksa, the incumbent, of rigging the vote.
Opposition parties have staged a series of protests to demand Fonseka's release despite attacks by police and government supporters.
Fonseka and Rajapaksa worked together to end the 25-year war against Tamil Tiger separatists last year, but fell out soon after.
Fonseka, who quit the military in November, used his fame to enter politics as an opposition presidential candidate.
The government said Fonseka had conspired against the president and would face a court-martial.
Sri Lanka's Supreme Court has admitted a petition filed by Fonseka's wife on grounds that his detention in military custody is illegal. The court on February 12 gave the government four weeks to give its reply to the petition.
Analysts say political unrest could affect the country's $40bn economy as it is poised to grow more than six per cent this year due to post-war economic optimism and high foreign investment.
Fonseka's detention has sparked international concern with the US, the EU and the UN, among others, who have asked Colombo to ensure that due process is followed and that democracy is not undermined.