The second of three High Court trials has been postponed until April 3 after the Post Office asked the Hon Mr Justice Fraser to step down.
The trials are the result of a group legal action brought about by 550 former post office staff, including Telford’s Tracy Felstead who say that a glitch with the Horizon computer system led to false accusations of theft or false accounting.
Miss Felstead, now 36, was jailed for six months in 2001 after being convicted of stealing £11,000, and is now trying to get her conviction overturned.
Former Shrewsbury sub-postmistress Rubbina Shaheen, 53, is also trying to get her conviction overturned, although she is not part of the High Court action. She was sentenced to 12 months in prison in 2010 over £43,000 which allegedly went missing from Greenfields Post Office.
Earlier this month, in the first of the three trials which looked at the contractual relationship between the Post Office, its sub-postmasters and workers, Mr Justice Fraser ruled in favour of the staff.
The second trial, which will look at the Horizon system itself, has now been halted after the Post Office accused the judge of bias.
If the Post Office’s application is granted, the trial will have to start again with a different judge. If it is rejected, the trial will resume next week.
The move came after witness Andy Parsons claimed that Mr Justice Fraser’s first ruling suggested hostility to the Post Office and that he had already formed an opinion on the matters yet to be discussed.
In the first trial, the judge ruled that the Post Office was guilty of “oppressive behaviour” when demanding sums of money that could not be accounted for by sub-postmasters.
The judge also accused Post Office director Angela van den Bogerd of deliberately misleading him, saying there were two specific matters where she “did not give me frank evidence, and sought to obfuscate matters, and mislead me”.
In a statement issued by the Post Office, a spokesman said: “We have reflected in great depth on the proceedings and detailed judgment from the first trial and will continue to consider all options.
“As part of this, we have made an application today for the sitting judge to be recused from the ongoing and upcoming trials. We are acutely aware of the significance of this application."
Miss Felstead, of Bournside Drive, Brookside, said the Post Office was simply trying to frustrate the process.
"I think it's a stalling tactic, to make us go away, but that's not going to happen," she said.
Alan Bates, founder of Justice for Subpostmasters Alliance which has brought the action, accused the Post Office of desperation.
"It seems to me that, as more truth about the way Post Office treated subpostmasters is exposed, the more desperate the Post Office becomes to defend the indefensible," he said.
Mrs Shaheen, who now lives in Worthen, is not part of the action, but along with Miss Felstead she is seeking to get her conviction overturned through the Criminal Cases Review Commission.