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Alastair Campbell clashed repeatedly with Today programme presenter John Humphrys this morning as he denied lying and defended his "aggressive and combative" style of dealing with the media.

The former Downing Street director of communications was discussing the first volume of his memoirs, which is published today, containing his account of the time he spent working for Tony Blair.

Mr Campbell has been accused of introducing a climate of spin into the Prime Minister's relations with the media, and of presiding despotically over the way information was released to journalists.

Today he justified himself, saying that the reason he had taken tighter control of the Prime Minister's public relations was because the media itself had changed. He blamed the rise of the internet and of 24 hour news for forcing newspapers to take up partisan stances in which a politician was either a "hero or a zero".

He said: "I think the media has changed faster than any one of us would have predicted. The rise of the internet and 24 hour news have forced newspapers to be far more posturing.

"Once you could freely distinguish between tabloid and broadsheet newspapers. Now those distinctions have gone. The centre of gravity has shifted."

Under pressure from Humphrys, who reminded him that in his days as political editor at the left-leaning Daily Mirror he had been equally partisan in referring to the then Prime Minister John Major as a "useless little git", and that numerous government press officers had resigned after Mr Campbell went to Downing Street citing clashes over his management style, the former spindoctor admitted: "Possibly the change was in part because of me.

"When we came into power in 1997 perhaps we did just inhale too much of the positive stuff (being written about new Labour at the time) and just slightly think we could walk on water."

Mr Campbell said he "didn't really know" why he was accused of bullying journalists. "I have never been able to work out why I provoke such strong reaction. There have been points at which Tony Blair, Peter Mandelson and my partner, Fiona, have said that sometimes I underestimate how aggressive I come across when I try to make a point.

"To be absolutely frank, I have tried to work on that, and one of the reasons I left in 2003 was because I realised that I had become not just part of the story but actually an impediment to effective communication by the government."

Mr Campbell's book has been eagerly awaited for its version of the most controversial events of Mr Blair's premiership, including the publication of an intelligence dossier which explained the case for invading Iraq. Mr Campbell's role was to advise the Joint Intelligence Committee on how best to present the information in the dossier, but he was later accused of hardening up the language, changing qualifying words like "may" to the more assertive "is".

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