KUALA LUMPUR (Reuters) - Malaysia has rejected a petition seeking a royal pardon for jailed opposition leader Anwar Ibrahim, who is serving a five-year prison term for sodomy, his lawyers said on Wednesday.

The petition was filed by Anwar's family in February after his sentence was upheld by Malaysia's highest court. A pardons board said Anwar's sentence would be maintained but gave no explanation for the decision. "We would definitely be appealing on the leave rejection, and we will be asking for further information as to what is going on," said Mr Latheefa Koya, one of Anwar's lawyers.

Anwar, who once posed the greatest threat to Malaysia's long-ruling coalition, was found guilty of sodomising a former aide, a charge that he said was a politically motivated attempt to end his career.

He was head of a three-party opposition alliance that made stunning gains in the 2013 election, which for the first time raised the prospect of a genuine challenge to the coalition that has ruled Malaysia since independence in 1957.

Anwar was the ruling party's rising star in the 1990s until he fell out with then Prime Minister Mahathir Mohamad. His family and political party have voiced concern about his health and conditions in prison since his sentence was upheld.

His last legal option is to file a judicial review with the Federal Court. His lawyers said no decision had been made yet on such an application.

The rejection of the royal pardon came after a series of rallies organised by Anwar's Parti Keadilan Rakyat (PKR) after his imprisonment, although turnout was low at the latest rally last week.

Anwar's daughter, PKR member of parliament Nurul Izzah, was caught up in a recent crackdown by the government and police under Malaysia's Sedition Act. She was held in detention for one night for comments made in parliament that were deemed"contemptuous" of Malaysia's judiciary.