No economic impact assessment has been done on how Boris Johnson's new Brexit deal could affect Holyhead, a senior UK cabinet minister has told AMs.
Brexit planning minister Michael Gove said it was difficult to have one when "there are so many variables in play".
Plaid Cymru's Delyth Jewell asked how AMs could be expected to back the deal without knowing the impact on the port.
Mr Gove said "Holyhead will be in a stronger position than ever" if the deal is passed by MPs.
Ms Jewell called it "astonishing" that no impact assessments had been carried out for Holyhead, the UK's second busiest port.
The deal agreed between UK and EU negotiators involves new administration and checks on goods crossing the Irish Sea, to avoid such bureaucracy between Northern Ireland and the Irish Republic.
Giving evidence to the Welsh Assembly's External Affairs Committee on Thursday, Mr Gove said: "It's difficult I think to have an economic impact assessment because there are so many variables in play.
"It's important of course for all of us to look at the variety of different factors but no impact assessment can ever give us the unvarnished truth because by definition it is impossible to predict with certainty how something as multi-faceted as the UK economy will grow and develop in the future."
Mr Gove said he had visited Holyhead and looked at the impact leaving the EU with a deal could have on the port compared to leaving without a deal.
He said his government had "absolutely done everything possible to mitigate any adverse effects".
Mr Gove added: "If we have a no deal that will create challenges for Holyhead and that's why we believe this deal is in the best interests of the UK economy overall and all those who work in and around Holyhead as well."
The UK government has asked assembly members to give their consent to the legislation that will underpin the Brexit deal in UK law.
While not legally binding on Westminster, it is convention that laws passed by Parliament which have an impact on devolution are also agreed by the Senedd.
On Tuesday, Wales' First Minister Mark Drakeford said the prime minister had agreed a "bad deal for Wales", arguing the assembly should refuse formal consent for Mr Johnson's bill.