The Northern Ireland peace process would not have been possible without the leadership and courage shown by Martin McGuinness, Tony Blair said.
The former prime minister said of the ex-IRA commander: "Once he became the peace maker he became it wholeheartedly."
Mr Blair was in Number 10 when the Good Friday Agreement was signed in 1998, a landmark in the peace process.
He said: "I grew up watching and hearing about the Martin McGuinness who was a leading member of the IRA engaged in armed struggle.
"I came to know the Martin McGuinness who set aside that armed struggle in favour of making peace. There will be some who cannot forget the bitter legacy of the war. And for those who lost loved ones in it that is completely understandable.
"But for those of us able finally to bring about the Northern Ireland peace agreement, we know we could never have done it without Martin's leadership, courage and quiet insistence that the past should not define the future."
Mr Blair said he got to know the Sinn Fein politician well during the "arduous" negotiations and "by the time that extraordinary day arrived in 2007 after almost a decade of hard work where we could witness the - to my generation - incredible sight of he and Ian Paisley sitting down together in government, the transition of Martin to reconciliator was complete".
The former prime minister said "whatever the past, the Martin I knew was a thoughtful, reflective and committed individual", adding that he showed "no shortage of determined opposition to those who wanted to carry on the war".
He said: "I will remember him therefore with immense gratitude for the part he played in the peace process and with genuine affection for the man I came to know and admire for his contribution to peace."