Earlier this year Mr Jeyaretnam formed a new opposition party
Veteran Singapore politician JB Jeyaretnam has died of heart failure in a Singapore hospital, aged 82.
He was the first to break a government monopoly on power in Singapore when he won a seat in parliament in 1981.
He had been forced into bankruptcy over defamation cases won by the government but was planning a new run for office.
Dubbed the Grand Old Man of opposition politics, analysts said Mr Jeyaretnam was a thorn in the side of Singapore's Minister Mentor Lee Kuan-yew.
Born in 1926 in Jaffna, Ceylon (now Sri Lanka), Mr Jeyaretnam trained as a lawyer in Britain before making his home in Singapore.
He served as an MP from 1981 to 1986 and from 1997 to 2001.
His first victory, as standard bearer for the Workers' Party, came when he defeated the People's Action Party (PAP) of founding prime minister of independent Singapore Mr Lee.
He was returned to parliament in 1984 but in 1986 was found guilty of making a false declaration of his party's accounts and fined a sum which made him liable to expulsion from the legislature.
He was disqualified from sitting in parliament until 1991, and disbarred from legal practice.
The Privy Council in Britain ruled in 1988 that he had been wrongly disbarred in "a grievous injustice".
Mr Lee "appeared determined to drive him from political life" wrote Professor Michael Leifer in his Dictionary of the Modern Politics of Southeast Asia.
Not until 1997 did Mr Jeyaretnam try again; he re-entered parliament as a non-constituency member without voting rights.
Just over a year later he was again brought to court, on the charge of having defamed then prime minister Goh Chok-tong and ten other senior members of the PAP.
Although the court found in favour of the government it awarded damages at only one-tenth of the amount possible but on appeal, damages were increased and full costs imposed on Mr Jeyaretnam.
By May 2000, he was declared bankrupt for failing to keep up payments in another libel case.
He left the Workers' Party in 2001, and was discharged from bankruptcy in 2007.
This year he helped form the Reform Party to challenge the 40-year rule of Singapore by the PAP, saying Singapore had been "enslaved" by its rulers.
He said in April he planned to run in the next parliament election, due by 2011.
His death came just days before he was to appear in the High Court to seek an order that a by-election be held for a seat that is currently vacant, according to AFP news agency.
The opposition has long been marginalised in Singapore, where it complains of limited access to the pro-government mainstream media and restrictions on public assemblies.
The People's Action Party, which has ruled Singapore since independence from Malaysia in 1965, holds 82 out of 84 elected seats in Parliament.
The Straits Times website described Mr Jeyaretnam as "pugnacious", an "old warhorse" and "irrelevant".