Sigma Huda, a UN special rapporteur on people trafficking, faces charges under the military-backed caretaker government's anti-corruption drive.

Mrs Huda said the Supreme Court had withdrawn her permission to travel because she was a "security threat".

She was due to address the UN Human Rights Council in Geneva next week.

'Immunities'

"We have been advised that Sigma Huda has been prevented from leaving Bangladesh, where she has reportedly been charged under provisions of anti-corruption legislation in that country," the Office of the UN High Commissioner for Human Rights spokesman Jose Luis Diaz told the Reuters news agency.

There is a court case involved and that's why she'll not be leaving the country Bangladesh government

He said his office had requested clarification from the Bangladeshi authorities on the legal proceedings and charges against her.

The Bangladesh government had also been asked "how, in light of the convention on privileges and immunities of the UN... such proceedings allow for keeping her from attending to her duties as special rapporteur," he added.

Bangladesh's acting foreign secretary, Zahid Hussein, confirmed to the BBC that Mrs Huda would not be joining delegates in Switzerland next week, where she had been due to address the fifth session of the UN Human Rights Council.

"We'll be telling the United Nations there is a court case involved and that's why she'll not be leaving the country," he said.

Correspondents say Mrs Huda is accused of having wealth disproportionate with her income. She is on bail and denies the charges.

'Security threat'

In an interview with the BBC, Mrs Huda said Bangladesh's High Court had granted her permission to travel earlier this year and that she had been able to travel on three occasions "under different excuses", including once in her capacity as a UN special rapporteur.

After not being allowed to leave Bangladesh on 14 May, Mrs Huda was told the country's Supreme Court had rescinded the permission.

Mrs Huda said she doubted it had simply been a legal decision.

"It's not a matter of the court," she said. "The court had already given me permission to travel."

"It's the government which went to the Supreme Court and termed me a security threat to Bangladesh."

Mrs Huda said the authorities would not say what kind of security threat she posed.

"Am I the security threat, or is the government itself the threat?" she asked.

Her husband, Nazmul Huda, was communications minister in the government of Bangladesh Nationalist Party leader Khaleda Zia, which left power in October.

He is among scores of senior politicians and business leaders rounded up in the anti-corruption drive. He also denies wrong-doing.

Bangladesh's caretaker administration declared a state of emergency in January and postponed elections after months of political violence.

It says it will hold polls by late 2008, giving it time to deal with Bangladesh's endemic corruption.