JAKARTA (REUTERS) - Indonesia's main anti-graft agency is bracing itself for a flood of legal challenges from corruption convicts and suspects after a court verdict threw into question the legitimacy of its investigations, a senior official said on Wednesday.

A Jakarta court judge on Tuesday ruled that the agency's investigation of graft suspect Hadi Purnomo was illegal because the Corruption Eradication Commission (KPK) used independent investigators no longer with the police or the Attorney General's Office, media said.

The verdict is the latest blow to the popular agency, which has been severely weakened over the past four months by attacks from the police, the Attorney General's Office, the vice-president, and members of President Joko Widodo's own political party. "The impact of this verdict could potentially make graft suspects and even convicts file lawsuits, which would drain the energy and resources of the KPK," said acting commissioner Johan Budi.

He added the agency was preparing to appeal against the court ruling.

Indonesia consistently ranks among the most corrupt countries in the world, according to Transparency International.

Activists have raised questions about Widodo's anti-graft credentials after the KPK was forced to drop a high-profile investigation into a police general based on a similar court ruling.

The agency has since seen top-ranking officials suspended, a key investigator arrested, and a slew of lawsuits from corruption suspects hoping to be let off.