The Good Friday Agreement is at risk because of Brexit after the UK leaves the EU, former prime minister Tony Blair has told the BBC. UK and Irish membership of the EU was central to the 1998 deal, he told BBC Radio 4's The World This Weekend. Free movement on the border had been key to reaching an agreement, he said.
The Republic of Ireland's Tánaiste (deputy prime minister) Simon Coveney warned against a hard border becoming the collateral damage of Brexit.
Mr Coveney, who is also Ireland's foreign minister, told BBC One's Andrew Marr show that Northern Ireland and the Republic of Ireland were uniquely vulnerable and exposed to a potentially bad outcome from Brexit.
We cannot allow some kind of collateral damage or unintended consequence of Brexit to [be] the recreation of a border on the island of Ireland, he said.
Mr Blair said the prospect of a hard border posed real challenges to the peace process and it was difficult to see how the issue would be resolved.
The ex-PM, who helped orchestrate the Good Friday Agreement, said that the UK and Ireland's EU membership made it easy to appease nationalist feelings in 1998.
The free movement of people, goods and an open border between the Republic of Ireland and Northern Ireland was part of that expression that the island of Ireland was together, he said.
But there are concerns that Brexit could lead to a hard border like that seen before the Good Friday Agreement - for example, by reintroducing customs checks between the two countries.
He told the BBC's Andrew Marr: If the Irish and EU wish to impose a border that would be a matter for them, but they don't have to do it, it is a question of political choice.
Mr Blair said: If you end up with a hard border, obviously that causes tensions. It doesn't mean that you should abandon the Good Friday Agreement, but it poses real challenges to it.
He urged negotiators to overcome the conundrum of creating a hard border between the UK and the rest of Europe, while preventing one from re-emerging between Northern Ireland and the South.
Brexit negotiators have said that the Common Travel Area between the two countries, which predates the EU, will remain in place.