Sinn Féin has warned any attempt to introduce Irish Border controls would be “a major setback for the political process in the North”.

Sinn Féin MEP Martina Anderson was responding to former British chancellor Nigel Lawson, chairman of the Vote Leave campaign, who said checks would be needed along the Border in the event of Brexit to prevent illegal immigration.

Leading Conservatives on both sides of the Brexit debate have spoken of the possibility if the United Kingdom votes to leave the European Union on June 23rd."There would have to be Border controls, but not a prevention of genuine Irish coming in," Lord Lawson told the BBC's Andrew Marr Show.

Dangerous prospects

British energy secretary Amber Rudd, who wants the UK to remain in the EU, said reintroducing Border controls between the Republic and Northern Ireland could not be ruled out.

“We’d have to wait and see, but what that does highlight is the very dangerous prospects that are out there if we do leave the EU. The lack of certainty is what concerns me,” she told Marr.

Lee Reynolds, who was the DUP group leader on Belfast City Council, is the Northern Ireland regional co-ordinator for the Vote Leave campaign: "We had a common travel area with the Republic of Ireland before we both joined the European Economic Community and we can have the same after we leave the European Union."

Illegal movement

Mr Reynolds told The Irish Times there were measures that could be put in place "that don't necessitate an immigration post". "We already have the existing agreement that covers the British Isles, you've got the expanded and improved co-operation on cross-Border security and policing and the biggest thing is, if we vote to leave, we are able to send people who have entered illegally back," he said.

“This isn’t the first time the pro-Remain campaign has pushed this dodgy claim, so it’s not a surprise they keep regurgitating it.”

He added: "You'll be able to drive from Newry to Dundalk to get your petrol and you'll be able to drive from Dundalk to work in Newry."

Sinn Féin MEP Martina Anderson said any attempt to reinstate Border controls in the event of Brexit “would represent a major setback for the political process in the North” and “directly challenge the integrity of the Good Friday [Belfast] Agreement as an internationally binding treaty”.

"The comments by Nigel Lawson that if Britain opted out of Europe we would see the reinstatement of Border controls are out of touch with reality," Ms Anderson said.

Ulster Unionist leader Mike Nesbitt said: “There is no certainty about what will happen in the event of Brexit, but we know from bitter experience that it is practically impossible to secure the Border between Northern Ireland and the Republic of Ireland.

“And as it is likely that freedom of movement will continue to apply on the island of Ireland, the challenge will be to stop illegal immigrants moving into Northern Ireland from the Republic.

“The next question that raises is how will the UK government stop people illegally entering Great Britain from Northern Ireland?

“The hard border could be at the ports and airports at Cairnryan, Heathrow, Gatwick and all the rest. Equally, there is no certainty there will be free trade, and will customs posts return?”