SINGAPORE, Aug 18 (Reuters) - Singapore's High Court has declared an opposition politician bankrupt, the Straits Times newspaper reported on Saturday, a ruling that could cripple the tiny Singapore Democratic Party (SDP).

Chee Siok Chin -- sister of SDP leader Chee Soon Juan, and herself a senior member of the party -- was declared bankrupt together with two other party supporters after they failed to pay about S$24,000 ($15,700) in legal costs.

Under the city-state's law, she would be unable to run for parliament as a bankrupt, putting the SDP's future at risk.

Chee Siok Chin had incurred the costs after she challenged a move by police to disperse a peaceful protest calling for greater transparency in state institutions.

Public protests are rare in Singapore. Any public gathering of more than four people requires a police permit and a person convicted of unlawful assembly can be fined up to S$1,000.

Chee Siok Chin led a handful of SDP candidates in last year's general election. The party did not win any seats in parliament but did get 23 percent of the vote in the wards it contested.

Her brother, Chee Soon Juan -- the country's most vocal opposition politician -- was declared bankrupt in February after failing to make libel payments of S$500,000 to former Prime Ministers Lee Kuan Yew and Goh Chok Tong.