The Co-operative has insisted its bank did not require government support after a ratings agency warned it may need a taxpayer bail-out.
In a message to reassure customers and members, the Co-op said "we haven't sought nor do we need government support" and insisted it was taking action to strengthen its bank's balance sheet.
A downgrade from Moody's saw Co-op Bank's investment grade rating slashed to "junk" status - followed just hours later by yet another blow as the lender's chief executive Barry Tootell resigned.
He had been brought in to lead the group's failed deal to take over more than 600 branches from Lloyds Banking Group.
The group has been at the centre of intense speculation over its financial strength after pulling out of the bid, which is thought to have collapsed as the Co-op Bank struggled to fill a £1 billion shortfall in its reserves to cushion against potential future crises.
It has been working to address this with the disposal of assets, such as its life insurance and asset management arm to Royal London and plans to sell its general insurance business.
Moody's said the Co-op under-estimated the risks of the Britannia acquisition, especially against the backdrop of weak economic conditions.
The Co-op said it was disappointed by the downgrade, but stressed it had a "clean plan" to bolster its financial strength.
It said: "We have a strong funding profile and high levels of liquidity, which are significantly above the regulatory requirements. We do acknowledge, like the rest of our banking sector peers, the need to strengthen our capital position in light of the broader economic downturn and the pending introduction of enhanced regulatory requirements, and we have a clear plan to drive this forward throughout the coming months."
© 2013 Press Association