The former boss of struggling lender the Co-operative Bank said he quit because superiors ignored his fears that a massive deal to buy more than 600 branches from Lloyds would be a disaster.
Neville Richardson told MPs he repeatedly warned Co-operative Group bosses it was taking on far too much and the bank could collapse - before quitting over the issue in July 2011.
And Mr Richardson insisted loans acquired with the Britannia Building Society - which he ran before it was acquired by the Co-op in 2009 - are not solely to blame for the bank's woes.
He said former bosses including Peter Marks, group chief executive until May, were determined to do the Lloyds deal - dubbed Project Verde.
But he said the self-styled ethical lender could well have been another Northern Rock if the Lloyds deal had gone ahead.
The Co-op Bank last week reported half-year losses of £709.4 million on surging bad debts and a bungled roll-out of a new computer system.
It is being forced to plug a £1.5 billion black hole in its balance sheet at the expense of bondholders, with hundreds of job losses expected. The Government could still have to step in and rescue the ailing lender if the fundraising is not successful.
Mr Richardson was boss of Britannia between 2002 and 2009, then chief executive of Co-operative Financial Services until his departure.
"If the Rochdale Pioneers (the Co-op's founders) had been at Thursday's results announcement I think they'd say that their successors have not lived up to the example they set," he told MPs on the Treasury Select Committee.
Mr Richardson said he raised his concerns with Mr Marks, chairman Len Wardle and deputy chairman Paul Flowers on a number of occasions in 2010 and 2011.
Mr Richardson stepped down by "mutual agreement" in mid-July 2011, after what he said was a final blunt warning about the consequences of trying to do too much at once.
"I used the word 'disastrous' and I used the words 'collapse of the banking system'," he told MPs. "These were pretty dramatic words."
In his first public appearance since quitting, Mr Richardson used parliamentary privilege to break a confidentiality agreement he signed with the Co-op.
Mr Richardson, who received a total payout of £4.6 million on leaving the group, said it was "the right deal at the wrong time".
He said: "The board and chief executive of the Co-op Group at that time did not accept my warnings and were determined to press ahead. That is why I stepped down."
Up until late April the Co-op was front-runner to buy the Lloyds branches, which Mr Marks claimed would propel it into the "premier league of UK banking".
But its ambitions crumbled when shortly after the deal's collapse the bank was hit by a ratings downgrade. In June it announced painful plans to plug a £1.5 billion capital gap - after City watchdog the Prudential Regulation Authority flagged concerns over its capital buffers.
Mr Richardson said he warned in July 2011 - when the Co-op was still only toying with buying the Lloyds branches - that the group was taking on too much.
He said the Co-op was risking "failure" as it simultaneously attempted the Lloyds deal, a multi-million pound IT overhaul, the sale of its life and savings businesses and a project to fully integrate Co-op Financial Services within the overall group.
He said he told his superiors that the business was "incapable with coping with all of this change at one time".
"This was increasing the bank from 100 branches to 1,000 branches in less than three years," he said.
The bank's recent first half results showed £496 million of impairment charges on soured loans.
But Mr Richardson insisted the Britannia was a success when it was taken over by the Co-op. He added Britannia loan losses only account for a third of the Co-op's impairments over the past 18 months.
The Co-op has launched an independent probe into what went wrong at the bank, and new management warn its turnaround will take four years.
Sir Christopher Kelly will head an investigation into its acquisition of Britannia and the failed Lloyds deal.