Alastair Campbell fears Brexit could unravel peace prosess in Northern Ireland
Tony Blair's spin chief at the time of the Good Friday Agreement says he fears for the future of Northern Ireland.
And Alastair Campbell, who was the Labour Prime Minister's Press secretary, said all the work done 20 years ago could unravel amid the current political stalemate.
Mr Campbell said: "Northern Ireland has undoubtedly come a long way since I was with Tony Blair pushing the peace process forward - but I worry when I look at things today.
"Brexit has thrown a big rock into the pond and if there is any solution to the issues over the border between Northern Ireland and the Irish Republic in the future, I have yet to see it.
"It's a conundrum that hasn't been solved and I struggle to see it being settled to the satisfaction of all.
"There is no devolved government and we need to get those institutions up and running as soon as possible.
"They need to talk about this. Politicians in Northern Ireland need to be working on this right now.
"As I see it, the DUP now have a stranglehold on power and they're enjoying being in the position of propping up the British Government. But that's an unsustainable situation.
"If we look at the abortion referendum in the Irish Republic and the fallout from that, that DUP power is what's preventing Theresa May from getting involved.
"I think we all know she wants to do something over abortion in Northern Ireland, but she can't.
"The DUP are too crucial to her Government to allow her to step in.
"Northern Ireland really needs to work out how it's going to be governed. The current situation is simply unsustainable in the long-term.
"There's all sorts of reasons why everything in Northern Ireland could unravel."
He was speaking ahead of a visit here this week for 'An Evening With Alastair Campbell', an event in the Belfast Book Festival.
"I haven't been over in Northern Ireland since I attended the funeral of Martin McGuinness and I'm looking forward to coming back, this time to Belfast," he said.
The main reason for his appearance will be to discuss his latest novel Saturday Bloody Saturday, which has strong local links. Former Deputy First Minister Mr McGuinness was used as a source for many of the background details.
The novel is set in the 1970s and involves Mr Campbell's love of football, but also draws on his experiences in Northern Ireland.
"There's an IRA hit squad involved, planning a major assassination in London, and it features a lowly north England football club - given my life-long support of Burnley," explained Mr Campbell.
"I enjoyed writing it alongside former footballer Paul Fletcher and I'm sure I'm going to enjoy talking about it."
His talk with BBC NI's Seamus McKee on Wednesday is just one of a number taking place across the city from June 6-16 as part of the festival.
It will be held at Windsor Park, where he will tell of his career in the media, working with Blair, and discuss his career as an author of 15 books.
"I usually find about a quarter of the time at these events I talk about the book, the rest of the time it's the political career," he said.
"I'm open to answering anything on the night."
Entry to the event is £14 (ticket only) or £20 (ticket and book).
For more information call 028 9024 2338 or visit www.belfastbookfestival.com.