Four candidates were certified on Thursday for Singapore's presidential election on August 27, setting the stage for the country's first contested polls for its head of state in 18 years.

Two potential bets were disqualified because they did not meet the stringent criteria for the presidency, including being chairman or chief executive of a company with a paid-up capital of at least Sg$100 million ($83 million).

The first and last time Singaporeans voted for their president was in 1993, when former deputy prime minister Ong Teng Cheong won. Before 1993 the president was selected by parliament.

It is a largely ceremonial post but remains significant because the president is the custodian of Singapore's foreign reserves, which stood at more than $242 billion in June.

Incumbent President S.R. Nathan, 87, who began a first six-year term in 1999 and was returned to office in 2005, is retiring from public service after running unopposed in both instances.

Among those certified by the Presidential Elections Commission is Tony Tan, a former deputy prime minister and ex-deputy chairman of the sovereign wealth fund Government of Singapore Investment Corp.

He had earlier resigned from the ruling People's Action Party (PAP) to run as an independent candidate in keeping with the non-partisan nature of the presidency, but is widely perceived to be the PAP-backed candidate.

The other candidates are former member of parliament Tan Cheng Bock, former insurance cooperative boss Tan Kin Lian and Tan Jee Say, the former regional managing director of asset management firm AIB Govett (Asia) Limited.

The presidential poll follows legislative elections in May which saw the opposition making significant gains by winning 40 percent of all votes cast, even though a block-voting system limited it to six of 87 seats in parliament.

Prime Minister Lee Hsien Loong conceded after the vote that it was a "distinct shift" in the country's politics and the cabinet has vowed reforms in immigration, housing and public transport policies.

The PAP has ruled Singapore since the former British colony became self-governing in 1959.