Identity checks at the border between the island of Ireland and Britain could be strengthened following a Brexit, an influential committee of MPs has said.
Screening already carried out by some airlines flying between Northern Ireland and the rest of the UK may be made more robust and extended to passengers at ferry ports like Holyhead in Wales and Stranraer in Scotland, according to the Northern Ireland Affairs Committee.
The committee warned that imposing controls at the soft land boundary between Northern Ireland and the Republic of Ireland, crossed by up to 30,000 commuters a day for work, would cause considerable disruption.
"An alternative solution might be to strengthen the border between the island of Ireland and the British mainland.
"There are fewer crossing points to enforce and it would be less disruptive as there are already checks in place.
"Some airlines flying between Northern Ireland and the rest of the UK already subject passengers to identity checks and these could be made more robust and extended to relevant ports such as Holyhead and Stranraer with relative ease."
A report by the Westminster committee - Northern Ireland and the EU Referendum - said there are nearly 300 formal and many informal crossing points between North and South.
A 2001 study estimated there were 18,000 daily cross-border commuters and one witness told the inquiry that figure could be as high as 30,000.
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The Northern Ireland Office has already warned arrangements guaranteeing free movement of people and goods with the Republic of Ireland could be threatened by Brexit.
Since the 1920s the UK and Ireland have operated a Common Travel Area which allows nationals of both countries to travel and live in each country without immigration controls.
They are part of an EU customs union which means there are no controls on goods.
The committee comprises MPs who support both the Leave and Remain campaigns.
Its report also identified the potential impact of a Brexit in key areas like the economy and agriculture.
It said: "In the event of a vote to leave the EU, it is imperative that Northern Ireland's economic priorities, such as gaining a good deal for agricultural and manufactured goods, are given due prominence by the UK Government in any subsequent negotiations.
"However, the likelihood of this cannot be guaranteed."
It said the Northern Ireland peace process was successful because of good relationships between British and Irish politicians, and MPs would expect that to continue regardless of the outcome of next month's referendum.
Committee chairman Laurence Robertson said: "Our intention was not to promote either argument - we have MPs on the committee from both camps - but to indicate what the significant effects of leaving the EU could be, what outcomes we can be sure of, and what we do not know."
New figures, meanwhile, show that net migration to Britain reached its second highest level on record in 2015.
The numbers have moved the issue of immigration firmly back onto the agenda in the debate about whether or not Britain should leave the EU.
The Office for National Statistics say the net migration figure hit 333,000 in 2015.
The net figure is the number of people moving to the UK for at least a year minus the number leaving the UK.
Net migration for EU citizens was 184,000.