A British parliament committee has said it is deeply concerned that no decision has been made on the location of customs facilities for the ports of Holyhead, Fishguard and Pembroke - with just 21 days left before the end of the Brexit transition period.
Holyhead is the second busiest roll-on/roll-off freight port in the UK and about half of the outbound freight from Dublin Port passes through it.
The confused state of customs preparation on the UK side could result in long delays for Irish truck drivers moving goods in and out of the country.
The Welsh government has prepared contingency plans in case facilities are not ready, including a plan to stack lines of trucks along the A55, which is the main road from the port that stretches across north Wales.
From 1 January, the UK will be outside the EU Customs Union and full customs procedures will apply to goods moving between Ireland and Great Britain.
The British government has decided to introduce customs and food safety checks in three phases between January and July mainly because the computer systems to process the extra customs paperwork are not ready and the physical facilities to carry out customs checks have not been built.
In a report published today, the Welsh Affairs Committee of the UK parliament said that even with a delay on introducing full scale customs checks until July, "there is an unacceptable level of risk that facilities will not be ready in either North or South West Wales for the full introduction of border checks and processes in July 2021".
The committee said that the decision on locating the Holyhead customs facility is taken by the UK government, while the Welsh government is responsible for the location of inland facilities to cover the ports of Pembroke and Fishguard.
It said delays in choosing a location for Holyhead have had knock-on consequences for the other ports.
Jeremy Miles, the Welsh government's Counsel General and Minster for European Transition told the committee that selecting a site for Holyhead was "the critical point for unlocking progress" for both 1 January and 1 July.
As an interim measure, the UK government is proposing to carry out customs checks on trucks arriving in Holyhead from Ireland in facilities in Warrington and Birmingham, which are 160km and 280km respectively from Holyhead port.
An initial proposal to site a customs facility on a park and ride site near the port was rejected by the local district council.
This week it was reported that the UK government has bought a transport cafe and associated truck park for conversion into a customs facility.
Local media reported that a Roadking truck stop, opened in 2015, is to be turned into a customs centre.
It has parking for 300 trucks. Staff there told a local paper that some 24 people are to be made redundant at the Roadking cafe to make way for the customs centre which is located on one of the roads out of Holyhead.
The director of port operations for Stena, which operates Holyhead port, told the committee he was concerned about the customs checks that will take place in Dublin port, where port facilities have already been built.
Ian Davies warned that if there is congestion at Dublin port it will "put out the schedules of the ferries that operate, which will then have a knock-on effect back in the Welsh ports, because when the freight goods check in at the Welsh ports for export to Ireland, they have to meet the pre-clearance for the Irish regulations".
While he told the committee he was not so worried about congestion caused by goods coming in from Ireland, he was worried about the consequential effects at Welsh ports in the event of disruption in Irish ports.
Wales is more exposed to trade with the EU than the UK average - 61% compared to 43% at UK level.
The committee report said "securing a trade deal with the EU is "is therefore of particular importance for the welsh economy".
It said a "no-deal" scenario risks having a disproportionately damaging effect on the Welsh economy.
The committee appeared to reject claims made by Prime Minister Boris Johnson that the UK will "prosper mightily" with or without a deal with the EU.
"We do not accept this would be a good outcome for the welsh economy, or that a a so-called "Australian style deal" is almost as good as the FTA that the UK is negotiating with the EU," it said.