Warrenpoint Harbour will require a 40,000sq ft facility for carrying out Brexit-related checks on goods from Great Britain, the area’s MP has said.
South Down MP Chris Hazzard said it would include HGV parking and offices.
But the harbour’s chief executive David Holmes said it is currently “unclear on the requirements, if any, for the construction of new harbour infrastructure”.
He added clarity from the government was “now an absolute imperative”.
Mr Hazzard, from Sinn Féin, acknowledged there is “still much confusion as to what exactly is being proposed; who is footing the bill; and when construction will begin”.
“The British government must urgently put its cards on the table and bring some transparency to this process,” he said.
At the end of the Brexit transition period in January, Northern Ireland will continue to follow EU rules on agricultural and manufactured goods, while the rest of the UK will not.
Additionally, the whole of the UK will leave the EU’s customs union but Northern Ireland will continue to enforce the EU’s customs code at its ports.
This will mean some new checks and processes for goods entering Northern Ireland and other parts of the UK.
The UK government said this will not require new customs infrastructure but it will need expanded infrastructure for regulatory checks.
Last week the MP for East Antrim, the DUP’s Sammy Wilson, said the Port of Larne was looking at using up to 14 acres of land for lorry parking and construction.
The chief executives of Northern Ireland’s ports are due to give evidence to a Stormont Committee on Wednesday.