Land in Anglesey could be turned into a parking site for lorries in the event of a no-deal Brexit.
The contingency plan, led by the Welsh Government, has been drawn up in case Holyhead port is unable to handle traffic after the UK leaves the EU.
It comes as MPs are expected to reject Theresa May’s Brexit deal.
The two sites on Anglesey are at the Roadking truck stop, near Holyhead, and land adjacent to the Mona airfield in the centre of the island.
The plan has been drawn up amid concerns that additional border checks in the event of a no-deal Brexit on 29 March could lead to traffic problems at Welsh ports connected to the Republic of Ireland.
In order to avoid disruption in that scenario, the UK government has said that it would minimise checks or simply waive through trucks from EU countries, such as Ireland.
However, the EU has said it would impose full controls on people and goods entering the EU from Wales and the UK.
On Monday, first minister Mark Drakeford told the assembly’s Brexit committee there was a “risk that that will create a backlog so that lorries that are destined for the republic will not be able to leave Wales.”
In order to deal with a potential backlog, Mr Drakeford said “there is space that can be used” at ports in Pembroke Dock and Fishguard – but “Holyhead is not in that position”.
He added: “So, there are, at a civil contingency level, discussions going on at Holyhead about what alternatives could be made…to make use of land that is not in the ownership of the port.
An Anglesey council source has told BBC Wales that the plan is to use Holyhead’s capacity of roughly 600 lorries in the first instance before using 200 or so spaces at the Roadking truck stop on the outskirts of the town.
If more space was needed, then lorries would be diverted to Anglesey Agricultural Show’s park and ride facility next to the Mona airfield.
The source said: “Beyond that, you’re looking at more sites in the private sector.”
As part of its planning for a no-deal Brexit, the Irish Government has said that it aims to “ensure that any potential disruption…is kept to a minimum” but that lorries travelling to Welsh ports and through the UK before reaching mainland Europe “may be subject to severe delays”.
About 70% of Irish cargo destined for UK and EU markets passes through Welsh ports – the majority of it through Holyhead, which is the second busiest port in the UK.
In the meantime, prime minister Mrs May is seeking to build support for her Brexit deal ahead of the vote in the House of Commons next Tuesday.
However, many of her own Conservative MPs are expected to join Labour, the SNP, the DUP and Plaid Cymru in opposing the plan.
First Minister Mr Drakeford will repeat at a Brexit business conference in Deeside on Friday his belief that a no-deal Brexit is “wholly unacceptable and must be ruled out as an option”.