Gordon Brown was so angry about emails sent by his former adviser slurring senior Conservatives that he could not speak, Damian McBride said today.
The aide quit No 10 in April when it emerged that he had sent emails making unfounded personal allegations about Conservatives, including party leader David Cameron, shadow chancellor George Osborne and his wife Frances and backbench MP Nadine Dorries.
In an interview broadcast on Victoria Derbyshire's show on BBC Radio 5 Live, Mr McBride said: "When I spoke to Gordon quite early that morning he was like anyone else - the first thing he wanted to know was 'Is this true? What have you said in these emails?'
"So I had to tell him the contents of the emails and it was one of the hardest things I've ever had to do.
"His reaction, as he said himself, he was, I think, probably so angry and mortified that he couldn't really speak, at least initially, about what I'd actually done.
"And that's characteristic of Gordon, despite the myths that go around about him losing his temper. I think when he's genuinely angry he can be sort of just very deadly silent. And that was the reaction I got.
"After that we then moved on to the process of me resigning."
Former civil servant Mr McBride emailed Labour activists about unfounded smear stories which could be spread on the internet.
The Prime Minister later said he accepted full responsibility.
Mr McBride said Mr Brown was "incredibly let down" by what he had done and quizzed him to find out whether anyone else knew about the messages and whether any Government computers had been used.
He said: "Everything about Gordon Brown in the years that I've been working for him, the sort of words I associate with him, integrity, and having incredibly strong principles, and also just an incredibly strong sense of discipline. Everything about these emails offended all those principles he's got, offended his sense of discipline."
But Mr McBride blamed journalists for releasing details of the emails themselves.
He said: "The one thing I can't apologise for, because it wasn't my responsibility, is putting these stories into the public domain."
The former Number 10 adviser is hoping to resume his career in the Civil Service, and said he thinks there is "all to play for" in the next General Election.
He said: "Momentum and the political sands can shift incredibly quickly and I think, from that point of view, I wouldn't rule out anything happening over the next year, no matter what the polls say at the moment.
"I would just say it looks to me like one of those elections where it's all to play for."
Mr McBride said he could not remember the contents of the emails when former journalist and lobbyist Derek Draper warned him that his account may have been hacked into.
He said: "When I read the emails I thought 'what the hell were you thinking writing this?"'
Mr McBride said he had not thought properly about the "quite nasty content" of the messages, which he had written to help Mr Draper set up a website to respond to right-wing blogs attacking the Labour Party.
This kind of behaviour was not characteristic of the way Mr Brown ran Downing Street, Mr McBride insisted, and his job did not involve smearing people.
He said: "If that had been the case then people would have been able to come out during that feeding frenzy in the week after I resigned and point to specific circumstances, specific instances where individuals had been smeared, attacked in a personal way."Reuse content