Lorries heading for Northern Ireland could quickly back up on to main roads if one encounters a problem with Brexit paperwork at Scottish ports, a senior harbour executive warned.
Regulations around the movement of goods from Great Britain are under discussion following withdrawal from the EU.
Engagement with Government customs authorities has been significantly ramped up in recent times, but concerns remain, port operators said.
Larne Harbour general manager Roger Armson said: “The distance between check-in and the main arterial route to the central belt (of Scotland) is very short.
“You do not need to get much of a delay to start backing traffic up on to those routes.”
Mr Armson is head of P&O’s Larne to Cairnryan ferry operation. He expressed concern if paperwork was not in order after a haulier turned up at the south-west Scotland port.
“If a unit arrives without a valid goods movement reference, there are no facilities in Scotland for that unit to park up.
“That is a bigger concern to me and something we are looking at very closely.”
About 200 lorryloads a day from the rest of the UK are needed to supply Northern Ireland’s supermarkets.
Stormont’s agriculture committee took evidence on Brexit planning from Northern Ireland’s port authorities on Thursday.
Maurice Bullick, compliance director at Belfast Harbour, said: “There has been a long engagement with HMRC.
“It is only in the last short while that the engagement with HMRC has really ramped up.”
Under the protocol contained in the Brexit Withdrawal Agreement, Northern Ireland will remain in the EU Single Market for goods when the transition period ends.
That will require additional regulatory checks for animal-based food products entering Northern Ireland from Great Britain.
Port operators confirmed new screening facilities for goods arriving in Northern Ireland from Great Britain will not be ready for the end of the Brexit transition period, but said contingency measures would be in place.
Negotiations between the UK and EU on a trade deal have covered areas like which goods are “at risk” of travelling from Great Britain into the Republic of Ireland via Northern Ireland, and should pay tariffs.
A large proportion of Foyle Port’s commerce in Northern Ireland’s north west crosses the Irish border a short distance away.
The Londonderry harbour engages in worldwide trade in commodities including oil, animal feed and fertiliser.
Chief executive Brian McGrath said work with HMRC had stepped up a gear recently following “generic” calls throughout the summer.
He said the organisation had begun to receive greater clarity from its political masters.
“Foyle Port’s experience in the last week has been of a much more focused and engaged response from them, which is ongoing and would give us some cause for optimism.
“We are now finally getting to the nitty gritty on the subject.”
He said political uncertainty over the Northern Ireland protocol’s “wellbeing” was of concern.
“In a black and white no-deal scenario we could lose 40% of our trade.”
He said it was important that trade continued unimpeded as it traditionally had.