ANTI-BREXIT posters put up across Belfast are "unlawful".
Sinn Féin put up a series of posters across the city ahead of last week's European Union summit in Brussels.
People Before Profit (PBP) also erected posters on lamp posts in response to Sinn Féin's campaign, accusing the republican party of being behind unbranded signs claiming the socialist party is in favour of a hard border.
Now the Department for Infrastructure (DfI), which is responsible for maintaining street lights, has declared the posters were "unlawful".
"The parties have been contacted in order to seek cooperation in having them removed and we have been given assurances that they will be removed in a timely manner," he said.
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"We will continue to monitor the situation before deciding if any further action is necessary."
Belfast City Council had indicated the posters needed planning permission.
A council spokeswoman said yesterday: "We are aware of this and our enforcement team are dealing with it".
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PBP MLA Gerry Carroll said his party were "forced" to put up their own posters because Sinn Féin's still have not been taken down.
"We call on DfI to explain why it continues to host Sinn Féin's posters attacking our organisation," he said.
He added: "We do not support a hard border, nor any border in Ireland, and have consistently called for a vote on a final deal in the north".
Sinn Féin MLA Fra McCann said Mr Carroll's party supported the UK's exit from the EU.
"His desperation to distance himself from the fact that People Before Profit campaigned for Brexit, voted for Brexit and acted as a cheerleader for Brexit increases by the day," he said.
