The legacy of Tony Blair's tenure as British Prime Minister has yet to be decided, according to a former aide who was written about his experiences in the administration of Tony Blair.

Alastair Campbell, a man who was once considered the second most influential figure in Britain while Blair was Prime Minister, praised the former British leader's achievements on a number of fronts. And he said that Britain's involvement in Iraq will be crucial in determining his overall effectiveness as a leader.

"When you talk about his legacy, obviously Iraq is an important issue and it's an ongoing issue and I don't think any of us can really make a final judgment as to whether that policy has been successful or not," Campbell told CTV's Canada AM.

Campbell was Blair's former communications director. He has just published "The Blair Years," a book based on diary excerpts he made from the time Blair became Labour party leader in 1994 until his resignation from his post as Blair's director of communication strategy in 2003.

Discussing Blair's Iraq strategy, Campbell's book suggests that Blair was the only leading government figure who did not display any major reservations about the decision to invade Iraq and that then-deputy prime minister John Prescott "looked physically sick" as the decision was discussed.

"I think (Blair's) close circle at various points all had our worries and concerns and doubts and it's not that he ever saw it as a black and white issue," Campbell said. "There were shades of grey throughout. It was a very, very difficult decision and he had to weigh it all up in the balance."

Campbell has been challenged for what he has allegedly left out of the book, specifically the long-standing tension and arguments between Blair and his successor Gordon Brown.

In a recent interview with the British Broadcasting Corporation, he said the reason for the omissions was to protect Brown from the ammunition the information may provide the opposition Conservatives.

Campbell's close proximity to Blair meant he was also behind some of Blair's speeches including one in particular where he famously referred to Princess Diana on the day after she died as 'The People's Princess.'

Campbell's book reveals that both himself and Blair were overwhelmingly charmed by the "extraordinarily beautiful" princess.

"Often there's no rhyme or reason to why one phrase will really register with public opinion, whereas another won't," Campbell said. "I think the phrase 'The People's Princess' just captured something about her that really spoke to how people were feeling."

Now that Blair has left 10 Downing Street, Campbell feels that Blair can be a success in his new role as a Middle East envoy.

"But it's not just down to him it's down to all the governments and all the people of that region," he said.