Irish Foreign Minister Charlie Flanagan has said a “heavily fortified” border between Northern Ireland and the Republic of Ireland would be “simply inoperable.”

Flanagan said that a border with tight security measures to prevent migrants entering Britain by the back door – that being via the Republic into the North - "won’t work."

Speaking to the BBC, Flanagan said: “It is absolutely essential that every effort be made to ensure the existence of what is an invisible border.

“The only visible sign is the change in the road signs from miles to kilometers as you cross from north to south.

“Any suggestion that there will be a heavily fortified EU frontier, or a heavily fortified border, be it for customs and trade on the one hand, or for security and immigration on the other, is simply inoperable.”

The issue of the border has been brought into focus once again following Britain's vote on June 23 to leave the European Union.

The majority of people in Northern Ireland, however, voted to remain in the EU.

The Irish government and all the political parties in Northern Ireland are determined that there will be no return to a “hard border” between north and south.

Former Northern Ireland Secretary of State, Theresa Villiers, who campaigned for a “Leave” vote, has also said the border should remain as it currently is.

“There would be, perhaps, some risk that non-Irish EU citizens might enter the UK over that land border.

“But the way you tackle people who come and work in the UK without the appropriate permissions is through measures such as cracking down on employing illegal workers.”

She added: “The idea of imposing, suddenly, a whole host of new border checks frankly isn’t practical and it’s certainly not desirable.”

Meanwhile, Sinn Féin President Gerry Adams has expressed his “deep disappointment” at what he called the “failure of the Taoiseach Enda Kenny to respect and uphold the vote of the people of the North to remain within the EU” during his meeting with British Prime Minister Theresa May in Downing Street last week.

“I agree with the taoiseach’s objective of minimizing the likely damage arising from the Brexit vote and of coordinating our response to it,” Adams said.

“However, there is no point in having that objective if, at his first meeting with the British Prime Minister, the taoiseach fails to defend the rights of citizens in the North to remain within the EU.

“There is also no point in the government spinning the line that there will be a ‘soft landing’ when the reality is that any restrictions on the border will have serious economic consequences.”