Mr Blair also offered to meet with the IRA army council in secret in a bid to keep the Northern Ireland peace process from collapse.
The revelations are made in a memoir by Jonathan Powell, chief of staff to Mr Blair for the duration of his decade as British prime minister.
Mr Powell also pinpoints in the book the IRA figure most central to the decision to disarm.
He writes that there would have been no decommissioning of IRA weapons without Brian Keenan, the army council member who served 18 years in jail for orchestrating bombing campaigns in Britain.
“Brian Keenan was at one stage the biggest single threat to the British state,” writes Mr Powell. “But he was also instrumental in bringing the IRA round to the political strategy and as the secret intermediary with John de Chastelain — the Canadian general in charge of the international commission on decommissioning — was the man who had achieved the decommissioning of IRA weapons.
“If he had been against it, it would not have happened. If he had died, it might have been impossible to persuade the IRA to trade the armalite for the ballot box.”
Extracts from Mr Powell’s book, Great Hatred, Little Room, are being serialised in the Guardian newspaper this week. Mr Powell recalls how, five months after Labour’s victory in the British elections in 1997, Mr Blair became the first prime minister in 80 years to meet with the leadership of Sinn Féin.
The meeting took place in Belfast in October of that year and was followed by a meeting in No 10 Downing Street in December.
“Alastair [Campbell — Mr Blair’s communications director] had even sent me a memo proposing we put off the erection of the traditional Christmas tree outside the front door of No 10 which was due to happen that day,” writes Mr Powell.
“He did not think we wanted a picture of Adams and McGuinness in front of festive decorations.”
Mr Powell recalls how, during the negotiations that followed over the years, Mr Blair offered to meet with the IRA army council to “try to reason with them himself”. Such a meeting meant Mr Blair becoming the first significant leader of a country to meet with a proscribed terrorist organisation.
“He was convinced that his remarkable powers of persuasion would succeed,” Mr Powell writes of Mr Blair, “but Adams would always say the time was not quite right, and maybe we should do it later”.
The negotiations led to the return of power sharing to Northern Ireland at a ceremony in Stormont on May 8 last year.
Mr Powell writes of that occasion: “I would have felt it to be an even more remarkable occasion had I realised the identity of the group of middle-aged men sitting in the next section along in the gallery.
“They looked harmless enough with their grey hair, but they were in fact the high command of the IRA, who between them had served over 50 years in jail and been responsible for more than 1,000 deaths. It was only after the ceremony that we discovered who they were,” he writes. “Each of the key IRA figures were there, including the quartermaster general, the military commander in Belfast, the head of intelligence and the chief ideologue — all sitting in the gallery just a few feet away from Bertie Ahern and Tony Blair.”
The book will be published on Thursday.