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Writing in his eagerly-awaited memoirs, Mr Blair said he took "horrendous chances" over what he told each party the others had agreed to.

The truth was on occasion "stretched past breaking point" in 2006, when talks to restore the power-sharing institutions were deadlocked between Sinn Fein and the DUP.

Around 8,000 words of Mr Blair's memoir 'The Journey' are devoted to Northern Ireland.

He defends "bending" the truth by saying it was necessary to keep the peace talks alive.

"Politicians are obliged from time to time to conceal the full truth, to bend it and even distort it, where the interest of the bigger strategic goal demands it be done," he writes.

"Without operating with some subtlety at this level, the job would be well-nigh impossible."

DUP East Londonderry MP Gregory Campbell says the revelation will not come as news to most Unionists.

"When negotiating with Tony Blair our emphasis always lay on actions not words," he said.

"We never accepted words because we knew how easily they had been ditched in the past.

"Indeed, that is why we ensured upfront delivery by the Government and Sinn Fein before we entered government."

However UUP leader Sir Reg Empey said the comments reveal that the St Andrew's Agreement was "built on lies".

"It has always been clear to some of us that the deal emerging from St Andrews was one done in haste and for the wrong reasons," the Ulster Unionist said.

"The Belfast Agreement was subject to a referendum in which the people of Northern Ireland were able to cast their judgment on what we negotiated.

"St Andrews was not like that, and we are suffering the consequences of having a deal done behind such smoke and mirrors.

"The DUP, sadly, had their eye wiped in part to satisfy Ian Paisley's vanity. We are living with the very real mistakes that were made in the then Prime Minister's approach."

TUV leader Jim Allister, who opposes power-sharing with Sinn Fein, says the St Andrews agreement is "not worth the paper it is written on".

"These memoirs give a glimpse into the deep immorality which lay at the heart of the 'peace process' and how some Unionists, who prided themselves on their prowess, were conned," he said.

"Now it turns out Blair lied to Paisley about what Adams had agreed to, resulting in a pledge of office not worth the paper it is written on."

Former SDLP leader Mark Durkan said: "Tony Blair knew that I didn't believe a lot of what he told me at the time.

"He didn't like it when you told him you didn't necessarily believe any of it.

"He would often say: 'Do you not trust me?' I would say:' Even if I wanted to trust you I couldn't rely on you."

Mr Blair also says the deal in 2007 almost collapsed over the choice of table for a key meeting at Stormont.

He explains that the DUP wanted the parties to sit opposite each other, "to show they were still adversaries", while Sinn Fein wanted everyone to sit beside each other "to show they were equals".

A deal was only reached, Mr Blair says, when a Downing Street official suggested the parties sit around a diamond-shaped table.

"Would it have broken the deal if it hadn't of happened? Probably not", DUP North Antrim MP Ian Paisley Jr.

"But it is one of those details, there are many details in all of this which at the end of the event now seem bizarre but at the time...kept the process moving."

Mr Blair also recollects an incident which occurred before the Assembly elections in 1997.

"I remember before the 1997 election a leading Orangeman describing me as unfit to be prime minister because my wife was a painted jezebel who claimed her allegiance to Rome," he writes.

The former Prime Minister also speaks of his relationship with his former chancellor Gordon Brown in the book.

He describes Mr Brown as "maddening" and "difficult", but also praises him for being "strong, capable and brilliant".

He adds that Mr Brown lost the general election in 2010 because he deviated from the New Labour message.

© UTV News