The first formal border checks between Britain and Ireland in more than 80 years have been proposed by the two governments.
London and Dublin outlined plans for identity checks on travellers which could involve a full passport inspection.
A Home Office consultation paper said it was putting forward "a substantial change" to the passport-free zone known as the Common Travel Area (CTA), which was set up in 1925 after Irish independence.
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 was "no intention to introduce fixed border immigration controls on the land border between the Republic of Ireland and Northern Ireland".
The plans do not cover travel between Northern Ireland and mainland UK, which will be the subject of a further set of proposals later this year, a Home Office spokeswoman said.
The document said: "We intend to carry out checks on passengers and their documents on sea and air routes arriving and departing from the UK from the UK to the Republic of Ireland for border control purposes. The introduction of routine immigration checks represents a substantial change to the UK's operation of the CTA."
Checks could involve examination of a passport or another form of ID for CTA residents - in other words, British and Irish nationals - to establish they are free to travel within the zone.
CTA residents and foreign passengers could be separated on arrival into channels similar to those already in place for European passengers on international flights.
The paper also proposed drawing up new laws to allow searches of vehicles travelling to and from the Republic. Airlines and ferry operators could be hit with fines if they carry passengers to the UK who do not have appropriate documents, it added.
