Ferry passengers travelling between Britain and the Republic of Ireland now need to show a valid passport before boarding.
Previously, passengers were not always asked to show a passport but had sometimes been required to have another form of photo ID.
The changes will affect about two and a half million passengers, including car and lorry drivers sailing from Holyhead, Fishguard and Pembroke Dock.
Ferry companies Stena and Irish Ferries say they are making the change from Monday to help them comply with UK government rules on passenger lists.
One tour operator said he had already received cancellations because of the ferry operators’ new policy.
