The first formal border checks between the UK and Ireland in more than 80 years have been proposed by the two governments.
London and Dublin outlined plans for identity checks on travellers that could involve a full passport inspection.
A British Home Office consultation paper said it was putting forward a substantial change to the passport-free zone known as the Common Travel Area, which was set up in 1925.
Immigration checks between the Republic of Ireland and Northern Ireland will also be stepped up to catch non-British and non-Irish nationals who travel illegally between the two countries, the paper said.
But it insisted that there is no intention to introduce fixed border immigration controls on the land border between the Republic of Ireland and Northern Ireland.