A senior judge has said she was pressured into sentencing opposition figure Moise Katumbi to a three-year jail term by Congolese authorities seeking to thwart his presidential ambitions.

Katumbi, seen as the leading challenger to President Joseph Kabila in elections due later this year, was last month found guilty in absentia of fraud in a real estate deal.

The ruling was handed down by a three-judge tribunal but in a letter seen by AFP on Wednesday, the presiding judge said officials had harassed her into signing the verdict and even threatened her with arrest if she did not comply.

"I was forced... to sentence Mr Moise Katumbi," Chantale Ramazani Wazuri wrote in the letter addressed to the government, the European Union, the African Union and UN representatives.

She said the goal was to ensure Katumbi would be ineligible to take part in the upcoming polls, for which a date has yet to be set.

She accused the head of DR Congo's intelligence services, Kalev Mutond, and judicial authorities in the country's second city Lubumbashi of putting "physical and moral" pressure on her to "go beyond" the scope of the law.

The dissenting judge has not been seen at work since Monday, one of her colleagues in Lubumbashi, a Kabila stronghold, told AFP.

Communications Minister Lambert Mende said the judge had left for Belgium, where she is now hoping to win asylum. He added that the letter would make no difference to Katumbi's case.

"This manoeuvre will have no effect on the judgement against Mr Katumbi," he said.

Katumbi, 51, a football magnate, also faces charges of hiring foreign mercenaries as part of an alleged plot to overthrow Kabila.

He left the Democratic Republic of Congo in May to undergo medical treatment in South Africa and has since settled in London. He insists however he will not withdraw from the presidential race.

READ: DR Congo opposition warns 'even worse crisis' over Kabila uncertainty

The justice minister has said that Katumbi will be jailed as soon as he arrives back in the country.

Congolese opposition parties — who fear Kabila plans to postpone the election in a bid to cling to power — said last month they had forged an alliance to demand that he quit when his term expires in December.

Kabila has been in power since the assassination of his father Laurent-Desire Kabila in 2001.