Jakarta (AFP) - Clandestine hotel room sex, money laundering and huge bribes to import beef evokes a seedy, criminal underworld rather than conservative politicians in the world's most populous Muslim nation.

But they all feature in a racy scandal that has shattered the clean image of Indonesia's biggest Islamic party and could further damage already-unpopular Muslim parties at national polls next year.

"The scandal... has given Islamic parties as a whole a bad image," said Umar S. Bakry, from pollster Lembaga Survei Nasional.

The controversy that has shocked the country peaked last week when an anti-corruption court sentenced the disgraced former president of the Prosperous Justice Party (PKS) to 16 years in jail.

Luthfi Hasan Ishaaq was found guilty of bribery and money laundering after accepting kickbacks from firm Indoguna Utama in return for pressing the PKS-controlled agriculture ministry to increase the company's beef import quota.

Two executives from the company had earlier been jailed over the case, dubbed "Beefgate" by local media, which has given blanket coverage to a scandal of enormous proportions even by the standards of graft-ridden Indonesia.

Ishaaq, who resigned as president of ruling coalition member PKS when the scandal emerged, has said he will appeal the guilty verdict against him.

During their probe, anti-graft investigators uncovered juicy details that tarnished the clean, pious image the PKS has sought to cultivate.

They seized six cars from Ishaaq and prosecutors accused the 52-year-old of trying to hide his marriage to one of his three wives, whom he wed last year when she was still a teenager.

But an arguably bigger figure in the scandal is Luthfi's close aide Ahmad Fathanah, jailed for 14 years in November, who was a key middleman in efforts to get Indoguna's quota increased.

His arrest in January kicked the scandal off in dramatic fashion -- anti-corruption agents caught the married man in a raid in a Jakarta hotel with a naked college student.

Fathanah had just collected bribe money and the student later admitted he paid her for sex.

He was found to have laundered his bribe money by giving gifts, including cars and diamonds, to 45 women, including an adult magazine model and several celebrities.

The PKS plays down the scandal and insists it is still on track for a strong result at legislative elections in April.

But independent polls in recent months show the party is receiving far below the almost eight percent it garnered at elections in 2009, and there is much public anger towards it.

"PKS is such an absolute disgrace, anyone who votes for or supports this party must be either totally delusional or incapable of independent thought," said a recent comment on the website of the Jakarta Globe newspaper.

"Beefgate" has scotched the party's recent efforts to reinvent itself by moving away from a purist Islamic agenda and presenting itself as a clean organisation as others were battered by graft allegations -- President Susilo Bambang Yudhoyono's Democratic Party in particular.

And the controversy risks affecting all Indonesia's Islamic parties, which were already struggling, analysts warn.

The five main Islamic parties, including the PKS, won a combined total of more than 25 percent at the 2009 legislative elections. They range from moderate groups to more extreme ones that want to introduce Islamic Sharia laws.

While the parties expected their share of the vote to continue the same downward trend of recent years, the PKS scandal means the fall is likely to be steeper and swifter, said Bakry from the Lembaga Survei Nasional.

He cited a recent LSN survey in which 42.8 percent of respondents said they expected the groups' popularity to fall and only 21.6 percent said they expected them to win more votes.

It is just another sad chapter in the history of political Islam in Indonesia, the world's most populous Muslim-majority country.

Islamic parties have seen their support erode gradually in recent years due to their own shortcomings and the greater appeal of the major, secular-nationalist parties, such the Democratic Party and the Indonesian Democratic Party of Struggle.

Experts point to poor organisation, infighting, previous corruption scandals, and a feeling among even conservative Muslims there is no longer an obligation to vote for a party describing itself as "Islamic".

"Years ago if you were a pious Muslim you voted for an Islamic party but now it's not the case," said Greg Fealy, an Indonesia expert at the Australian National University.

Most voters, he added, now opted for parties with a solid track record of running the country.