Northern Ireland's Communication Workers Union is considering balloting its members on industrial action less than a week after an 18-day dispute among Belfast postal workers was called off.
Shop stewards have claimed staff who took part in the unofficial action have been victimised since they returned to work.
But Royal Mail denied the allegations and branded the union irresponsible.
The Federation of Small Businesses said its members were still suffering from the mammoth backlog left by the strike, while the Ulster Unionist Party urged the Government to get involved before the dispute escalates.
John Farnan, of the CWU's national executive, said by Wednesday both parties expected to agree the identity of the body which will carry out an independent review of industrial relations at the Belfast depot. But he said the issue of discrimination remained a sticking point and will be the focal point of further talks.
"What has been happening is that people who did not take part in the strike have been treated more favourably," he claimed. "We are hoping that might be eliminated on Friday but if it is not then the ballot will still go ahead."
Royal Mail last week said it will take up to a month to clear seven million letters and parcels delayed by the strike. Extra staff have been hired and other distribution centres across the UK are being used to help shift the massive backlog.
The unofficial action began on January 31st amid allegations of management harassment and bullying at a Belfast depot.