A judge halted Ugandan opposition leader Kizza Besigye's treason trial on Wednesday after the defence said a star prosecution witness they accused of being coached via a hidden radio had fled the country.
Judge Vincent Kagaba gave prosecutors one day to produce witness Jennifer Aryemo to disprove the defence's charge.
Besigye's lawyers say Aryemo, 34, lied about meeting northern rebels at his request.
"We are reliably informed that this woman has been testifying under duress and that she has actually fled the country," defence lawyer John Matovu said.
He did not say to which country she had gone.
A week ago, the defence stunned the packed High Court when they accused her of smuggling in an earpiece and radio so that she could be coached during her testimony. She denied it, but failed to take the stand on Wednesday.
Prosecutor Simon Byabakama earlier said a policeman had told him Aryemo was sick.
"If she is sick, she is simply sick of questions," David Mpanga, another defence lawyer, told the court on Wednesday. "Her evidence should be expunged from the record."
Supporters of Besigye, who lost February 23 elections to President Yoweri Museveni, say the treason charges are politically motivated. Western governments say his arrest casts doubt on Uganda's image as a democracy and some have cut aid.
Aryemo told the court Besigye sent her to propose an alliance with the rebel Lord's Resistance Army in 2001. Besigye and 22 fellow suspects deny treason.
The retired colonel says the charges were meant to undermine him ahead of February's polls, and his supporters point to separate rape charges against Besigye that were quashed last month by a judge who called them "crude and amateurish".
At a news conference later, Besigye said he had received information the government was planning to fabricate more evidence and charge him afresh, preferably before a military court martial, now the treason trial appeared to be faltering.
"Sources revealed plans to stage manage an interception of weapons in or around Kampala where the so-called "victims" would confess to being my collaborators," he told reporters.
"This is because it is becoming increasingly clear none of the current bogus charges will stand up in a civilian court."
Government officials were not immediately available for comment.