Sinn Fein wants Northern Ireland to remain in the EU’s single market and customs union when the UK leaves the bloc as planned next March. The issue of an Irish border has been a major sticking point in Brexit negotiations.

Sinn Fein leader Mary Lou McDonald has warned that Britain’s departure from the bloc would make Irish unity a "more imminent reality" than ever before, and that for Northern Ireland to stay in the European single market after Brexit was “the only way” to secure its two decades-old peace accord.

She told Sky News that Tory in-fighting over the issue had demonstrated their "utter indifference to Ireland," adding that Brexit would lead to a referendum on reunification of the island.

Prime Minister Theresa May and the pro-Brexit Democratic Union Party (DUP) have steadfastly ruled out treating Northern Ireland differently from the rest of the United Kingdom.

The Sinn Fein leader also warned that the re-introduction of any kind of border on the island after Brexit must be avoided.

“To impose such a measure would be catastrophic in terms of our commerce, in terms of our access to services… in terms of how people live their day-to-day lives,” Mary Lou McDonald emphasized.

"Brexit is not of our making. We didn't want Brexit. In fact, the people in the north of Ireland voted against Brexit. They voted to remain,” she added.

McDonald criticized recent comments by several British lawmakers suggesting the 1998 peace accord — known as the Good Friday Agreement — should be revisited.

"Any notion that the Good Friday Agreement has served its purpose and should be put through the political shredder is grossly irresponsible," she added.

Sinn Fein is an Irish republican political party dedicated to the reunification of Ireland and an end to British jurisdiction in the north of Ireland.

The 1998 Good Friday Agreement ended deadly violence and gave partial self-rule to Northern Ireland.

READ MORE: Theresa May Rules Out NI Hard Border Between Ireland or Great Britain

EU and British officials have been deadlocked in talks regarding Ireland and Northern Ireland. Many speculate that a ‘hard Brexit' will create a hard border between the Republic of Ireland, an EU member, and Northern Ireland of the United Kingdom, with the Good Friday accord potentially at stake.