Byrne, a former Labour Party chief press officer, told PRWeek that Brown's top media handler was behaving in a ‘pathetic' fashion and that the party urgently needed to bring in Alastair Campbell-type figure.
Until this week, Byrne has refrained from public criticism of 'Team Brown'. But the PR agency boss decided to speak out in response to stories this week about Brown and the other G8 leaders feasting on eight courses after discussing world food shortages.
Byrne initially vented his anger on his Byrne Baby Byrne blog. In a post on July 8, he wrote: ‘Quite what prompted the incompetents - as they clearly are these days for all their fat salaries and big job titles and egos - in the No10 bunker to have the PM telling us to eat up our crusts one day and be photographed waving a glass of wine around the G8 dinner table as he tucked into the conger eel the next is beyond this simple communications guy's understanding.'
Speaking to PRWeek, Byrne elaborated on his comments. He said: ‘There's no way that these mistakes would have happened when Alastair was there because he had news sense, he was street-smart. From what I can see there is no-one at Downing Street from Brown downwards who has any street smart.'
He added: ‘What is the message coming out of Downing Street that defines his premiership? Nobody knows. They need to have a very clear political strategy which then translates in to a clear communications strategy.
'Is it that the communications people are not in the driving seat? Are they too busy fighting among themselves? Or are they just not good enough?'
Byrne singled out Brown's special advisor on press Damian McBride for criticism and suggested that Tory comms chief Andy Coulson was out-manoeuvring McBride.
He claimed: ‘McBride is just phoning up people and shouting at them. That's all he does. It's pathetic and they are losing friends left, right and centre in the media. They do need to get an Alastair Campbell or Andy Coulson-type figure in as at the moment there's no-one who understands the media.'
Downing Street did not respond to Byrne's comments when approached by PRWeek. McBride was on the Prime Minister's plane flying back from the G8 summit as PRWeek went to press.
Byrne was the Labour Party's chief press officer from 1988 to 1996. He joined Shandwick Consultants in 1996 and was made CEO of Weber Shandwick UK and Ireland in 2003.
Bell Pottinger Public Affairs chairman Peter Bingle said Byrne's comments were significant. ‘It's troubling for Brown if people are like Colin are speaking out publicly,' he said.
‘Colin is one of those who signed a lifelong pledge of loyalty and now he's being critical. If Charlie Whelan speaks out too then Brown may as well pack up his bags and go home.'
Whelan, Brown's former press secretary and a staunch defender of his old boss, could not be reached for comment as PRWeek went to press.