Two post offices have been earmarked for closure with the threat of up to 13 job losses.
At risk is the branch at Shaftesbury Square in south Belfast and a second in Castle Mall, Antrim.
The proposal is part of a UK-wide plan to franchise more large Crown post offices and cut costs.
South Antrim UUP MLA Adrian Cochrane-Watson said the move could be disastrous for Antrim town centre.
"I come from Antrim and am proud of the town but this could be the death knell for Castle Mall and the centre," he added.
"One by one, we have seen retailers pull out of Castle Mall to the point there is very little left there. It's sad because Antrim town centre has really struggled to attract inward investment.
"We have had £2m spent on the high street on a public realm scheme, and unfortunately I don't think it's attracted more businesses in to the town centre."
A spokeswoman from the Post Office last night said: "Regrettably, Shaftesbury Square Post Office branch is not sustainable.
"We cannot look to the taxpayer to subsidise branches that lose money so we must address this.
"The area is well served by Post Office branches, with Belfast City Crown branch just over a mile away and a further 11 branches within a two-mile radius.
"These branches will be able to absorb extra business and the proposal will help ensure long-term viability of Post Office branches in Belfast.
"In Antrim, we announced three years ago that we planned to franchise. We are continuing to make every effort to do this, but we cannot sustain the branch indefinitely and we are therefore making plans for its closure."
The spokeswoman said the Post Office was currently drawing up detailed proposals and will announce a period of public consultation in due course.
She also said the Post Office would support the 13 employees - seven in Belfast and six in Antrim - affected by the plan.
However, the Communication Workers Union (CWU) said the move was "condemning the service to decline" and pledged to campaign against the proposal.
CWU assistant secretary Andy Furey added: "This is devastating news for our members. It is death by a thousand cuts. The proposals disregard the wealth of expertise, experience and sense of pride in public duty which is shared by Post Office employees."
"Post Office Ltd should never have been separated from Royal Mail. It should be developing services with Royal Mail, and Crown Post Offices should be a one-stop shop for mail, banking, financial and retail services.
"This trusted public service is being destroyed by lack of imagination and a dash for cash when what it needs is innovation.
"Downgrading the crown offices is blatant privatisation and the Post Office is getting away with not going to Parliament for discussion on this by MPs.
"The closure will have negative effects on high streets, with the businesses left behind suffering and the area going into decline. The only thing driving this move is cuts to make profit."
Unite union officer Brian Scott said people in south Belfast and Antrim "will be quite rightly angered by this news that their Crown Post Office will close".