The shadow culture secretary said the regulator had failed to protect the interests of licence fee payers.
The BBC Trust "has proved itself a busted flush" as increasingly fractious charter renewal negotiations between the Government and the BBC gather pace.
Shadow secretary of state for culture, media and sport Chris Bryant MP told a packed RTS event Tuesday evening that the public broadcaster's regulator had markedly failed in its duty to protect the interests of licence fee payers since the UK general election.
The Labour MP for Rhondda characterised Chancellor of the Exchequer George Osborne's move to pay for free television licences for the over-75s in exchange for the promise of a possible CPI linked increase in the licence fee from 2017 as a "raid" on BBC coffers and a "stick up job".
Bryant argued that the BBC was now having to make political decisions and said he was "mystified that the whole of the Trust didn't resign on mass" over the issue.
"I'm sure they had a gun to their heads and they chose to get shot in the leg. The trouble with that is a leg wound can be fatal, " Bryant said, noting that the costs to the BBC would only rise over the years.
The shadow minister said: "The chair of the Trust issued a letter after the event objecting to the process, but since it was exactly the same licence fee process as was used in 2010, you might have thought they’d have made clear their objections in advance and in public."
The RTS audience of industry heavyweights included Richard Ayre and Stewart Purvis, the former chief executive of ITN who is one eight panel members appointed by culture secretary John Whittingdale to advise him on the charter process.
Ayre, a former BBC controller of editorial policy and a current trustee of the BBC, wasted no time during the Q&A session to say why neither he nor the board tendered resignations over the government's move.
He said there was no way the government would back down on its decision and so it was better to stay and take the government on, on behalf of the licence fee payers.
Ayre said: "For the next six months the BBC Trust and the BBC executive are going to have to fight a charter renewal battle. The BBC Trust is charged with representing the interest of licence fee payers. That's why we stayed, that's what we'll do."
Ayre also took the opportunity to thank Bryant for 95% of his wide-ranging, passionate and heartfelt defence of the BBC and its importance to the UK and beyond.
A former BBC high flier - he was formerly head of European affairs when John Birt was Director General - Bryant opened the event chaired by The Economist's Anne McElvoy accusing the government of a "bully crusade" against the public broadcaster.
"No one can be in any doubt now that the BBC is under siege," Bryant said. "My fear is that if the Government engages in a deliberate attempt to limit the BBC in the Charter renewal, the BBC will be a national irrelevance by 2027, little more than a PBS channel."
Bryant, describing the BBC as the fifth estate in the UK, "an intrinsic aspect of modern life" backed a CPI link in licence fee from 2017.
"Even if the BBC gets its CPI inflation rise as half-promised, it will still be shrinking even as sports rights’, movie rights’ and top talent inflation are racing ahead. And we shall all be the poorer for it."
Bryant, who describes himself as a "sometimes very critical friend" of the BBC said he hoped the public broadcaster would improve its record on diversity and address the governance issue by handing over regulatory duties to media watchdog Ofcom.
Purvis told the event from the audience not to buy suggestions in the national press that the Government was planning “root and branch” changes to the BBC.
He added: “The key point is wait for the green paper on Thursday. We’ll be in a better position to judge.”
Bryant said if the Government's aim is to cut the BBC down to size, "they do not carry the nation with them" in that quest.
"And I will fight them every inch of the way. I hope you will do too," Bryant said, reminding everyone there is 18 months left before the renewal.
The event was produced by Sue Robertson and Dan Brooke.
A full report will be published in the September issue of Television magazine.
Photos by Paul Hampartsoumian