Tony Blair has warned that a hard border between Northern Ireland and the Irish Republic would be a “disaster”.
The former UK PM said later he believed there was a “common desire” to make Northern Ireland a “special case” in Brexit negotiations.
An open border had done a “tremendous amount” for UK and Irish trade and must be safeguarded “as much as possible”.
The British and Irish governments have both said they do not want a return to customs posts on the border.
When, as part of Brexit, the UK leaves the EU’s customs union there will have to be some from of customs enforcement.
The EU’s negotiating guidelines call for a “flexible and creative” approach to the customs issue but no solid plans have yet been advanced by either the EU or the UK.
The EU’s chief negotiator Michel Barnier has told a joint session of the Irish houses of parliament in Dublin that the Irish border issue would be one of his three priorities in negotiations but emphasised that there would have to be some form of customs controls between NI and the Republic after Brexit.
Mr Blair, who was UK prime minister during the Northern Ireland peace process, was in Wicklow, Ireland to address a meeting of MEPs from the European People’s Party – the largest group in the European Parliament.
Speaking to RTE Radio before the meeting, Mr Blair said: “It really would be a disaster to have a hard border.”
But following the EPP meeting later he stressed that he believed there was a “common desire” to treat Northern Ireland as a “special case”.
He said he was “extremely anxious” to ensure Brexit did not damage the Good Friday Agreement – of which he was one of the architects.
“At the moment we have a common travel area where people can travel freely between south and north … on the island of Ireland. This is vital to maintain.”
“I think whatever disagreements I have with the British government over Brexit more generally, I think there is a real consensus across the British political system that we must do everything we possibly can to keep the present situation between the Republic of Ireland and Northern Ireland … as similar to what we have at the moment as we possibly can.”
Mr Barnier, who has been meeting business people in the Irish border region on Friday, told reporters he wanted to find solutions “without rebuilding any kind of hard border” and to protect and preserve the Good Friday agreement. “But we have to find a solution which is compatible with the single market”.
He said Brexit negotiations would be “very complex and difficult”. “This negotiation will not be only financial, legal or technical. In my view it will first [be] human, social and economic,” he said.
BBC Northern Ireland’s economics and business editor John Campbell said the free movement of goods was likely to be a key area of negotiations – and agricultural food companies in particular were extremely concerned about the potential for tariffs which could wreck their business.