LONDON — Twenty years after Northern Ireland’s landmark peace agreement, former British premier Tony Blair has recalled how close talks came to collapse -- and warned it risks being undermined by Brexit.

"I did not know if we were going to get an agreement until literally minutes before it happened," Blair said, ahead of a visit to Belfast to mark Tuesday’s anniversary of the 1998 Good Friday accords.

Blair remembers then US special envoy George Mitchell telling him it was "not going to work" when they entered the final round of talks involving the British and Irish governments and main Northern Irish parties.

The deal came off in the end, putting an official stop to three decades of violence that killed some 3,500 people.

However, Blair warned the deal is now complicated by Britain’s decision to leave the European Union, which risks reintroducing border checks between Northern Ireland and Ireland.

Britain and Ireland joined the bloc together in 1973 and are currently both members of the single market and customs union, which means there is no need for checks on travellers or goods.

Brexit "changes what has otherwise been the symmetry of relations between Ireland, the UK and Europe -- that is now broken if Brexit goes ahead", Blair told journalists in London.

He held back from warning of new violence, but cautioned: "Peace has to be worked on continually, so you should never think of it as guaranteed.

"The Good Friday Agreement will have to survive Brexit, and should survive Brexit, but it’s a complication."

 Takes time

The agreement was signed on April 10, 1998, which was Good Friday.

Blair will mark the anniversary alongside key players including Mitchell, former US president Bill Clinton, and former Irish and Northern Irish leaders Bertie Ahern, Gerry Adams, David Trimble and Peter Robinson.

The deal created a power-sharing executive between the majority Protestant unionists who wanted to stay part of Britain and the Catholic republicans who advocate reunification with Ireland.

There has been criticism that it entrenched divisions, and the two sides have failed to form a government since January 2017.

"If we hadn’t gone for a power-sharing process we just wouldn’t have had a peace," Blair insisted, adding: "These things always take time."

The former Labour leader, who was in office from 1997 to 2007, strongly opposes Brexit and backs a new referendum on the terms of the final withdrawal deal struck with Brussels.

He believes many Brexit supporters underestimated "practical issues... of which the most acute is Northern Ireland".

Ireland, the EU and Britain all oppose a so-called "hard border", but with Britain planning to leave the single market and customs union, it is not clear how this can be avoided.

Blair warned: "You are not going to make it disappear. I don’t think there is a way apart from staying in the single market and customs union." — AFP