Damian McBride worked for Gordon Brown at the Treasury before moving to No 10
Damian McBride, the Downing Street aide who quit over e-mail smears about the Tories, says Gordon Brown was so angry at the time he could hardly speak.
In his first broadcast interview since he resigned, the former special adviser told the BBC he was ashamed of the lurid e-mails he wrote from Number 10.
He left his post in April after a blogger revealed their contents.
But some of the stories written about him smearing Mr Brown's critics in the Labour Party were "ludicrous", he said.
The former adviser's job was to try to manage the way the press treated Mr Brown.
But when e-mails he wrote from his Downing Street computer making false and offensive accusations about senior Conservatives were revealed, he resigned, apologising for the "inappropriate and juvenile" content.
In an interview with BBC Radio 5 Live's Victoria Derbyshire programme, he insisted the e-mails had simply been stories, well known in Westminster among journalists, that he had noted down in an e-mail.
He said it was intended to help out his friend Derek Draper - who was planning on setting up a left wing blog to compete with "scurrilous" blogs which had become a "thorn in the side" of the Labour Party.
But he said as far as he was concerned the stories had "gone in the dustbin" when Mr Draper decided to dump the idea of the Red Rag blog.
Looking back at the content he had been "appalled" and admitted the emails had "quite nasty content" but denied it was characteristic of the way he did his job.
He said nobody else in Number 10 Downing Street, including Mr Brown, had known about the e-mails. The prime minister later apologised for the episode - Mr McBride had been one of his closest advisers.
Mr McBride said telling Mr Brown exactly what had been in the e-mails had been "one of the hardest things I've ever had to do".
"His reaction, as he said himself, was I think probably so angry and mortified that he couldn't really speak, at least initially, about what I'd actually done."
He said when Mr Brown was "genuinely angry" he went "deadly silent".
He added that when he said he knew he had to resign, Mr Brown "immediately agreed with that".
The former special adviser, who was nicknamed "McPoison", said the e-mails did not reflect the way Mr Brown operated and insisted his job did not involve smearing people.
He said there had been a media "feeding frenzy" after he resigned in which all sorts of things were laid at his door but he denied giving "attack briefings" against individuals.
"I was being accused of all sorts of ludicrous things that I had absolutely no involvement with," he said.
His special adviser status meant he had been allowed to give partisan - and sometimes strident - political briefings to journalists.
In the BBC interview, Mr McBride said his "McPoison" nickname was "inevitable" given his role working for Mr Brown at the height of the "tension" between the then chancellor and former PM Tony Blair.
"There was a lot of tension there ... That was always overblown to some extent but when that did come out and expressed itself in vitriolic briefings against Gordon ... then inevitably part of my job was reacting to that."
He accused Labour backbencher Frank Field and former Blairite cabinet ministers Charles Clarke, Alan Milburn and Stephen Byers of "vitriolic" comments and said he would not have been doing his jobs had he not responded "in some kind".
He also admitted mishandling coverage of a mooted general election in the autumn of 2007.
"If I had my time again I would have done that very differently," he said.
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