The Post Office has warned it could require a bailout of potentially hundreds of millions of pounds if it loses a major workers' rights court case in June.
The bailout could top £400million and would cover back pay for workers who are seeking employment rights and compensation going back more than 20 years.
The postmasters argue they should be considered as employed workers rather than independent contractors in a case similar to that won by Uber drivers earlier this year.
The Communication Workers Union (CWU) union is demanding they are considered as employees and are given benefits such as holiday pay.
© Getty60,000 Uber drivers face payouts after the Supreme Court rules they should be classed as workers with holiday pay
The claim stretches back 22 years, when the Working Time Directive was introduced.
If the Post Office loses at the Employment Tribunal in June, it could face a compensation bill of more than £400million, according to accountants.
While this case involves just 10 postmasters, a victory could lead to claims from its entire 8,000 workforce.
It comes just one week after 39 former Post Office workers were cleared of fraud and theft charges.
The employees - many of whom are no longer alive - were jailed following a Post Office investigation that found them guilty them of fraud - when in reality, it was down to a faulty IT system.
Many of these workers are now facing compensation payouts for damages caused in addition £58million paid to hundreds of other affected workers in December 2019.
It is understood the Post Office's chief executive, Nick Read, has warned the Business Secretary, Kwasi Kwarteng that losing the case could have a "material adverse impact" on its finances.
The Post Office's latest public accounts report warned the outcome of the tribunal, along with the claims for compensation made by the ex-postmasters wrongly convicted, represent a "material uncertainty" which could cast a "significant doubt on the company's ability to continue as a going concern."
It then noted the importance of government financial support if it was needed.
A spokeswoman for Kwarteng confirmed that the Government is aware of the legal proceedings.
The Post Office has received more than £1billion in loans and investment from the Government in recent years, but notched up a £305million loss last year.
Jo How, a director at tax advisory firm Blick Rothenberg, analysed Post Office accounts to assess how much the tribunal case could cost.
She said: "The exposure could be more than £400m and that's just in holiday pay. If sick pay and pensions are factored in, plus the ongoing costs of employing these 'workers', [it] could be considerably more."
A CWU spokesperson said: "The pay and benefits for postmasters are scandalous."
A spokesman for the Post Office said: "We take the issues raised by the claim very seriously."
The CWU claims the average pay of a postmaster is less than £20,000 a year for working between 60 and 70 hours a week, less than the minimum wage.