Sir John Vickers, who scrutinised banking regulation after the crisis, has warned reform is still unfinished, reports the BBC.
The system is "roughly halfway" to where it should be with the banks' capital buffers, he said.
The Bank of England has said capital requirements are 10 times those before the crisis for the biggest banks
But Sir John, who produced a landmark 2011 report, said he was "very disappointed" the regulator took the view the system had enough capital.
His comments come 10 years after the Bank of England was forced to provide emergency funding to Northern Rock.
The UK bank collapsed as nervousness about the US housing market caused problems for banks and governments around the world.
The Independent Commission on Banking, chaired by Sir John, recommended in 2011 that banks should have much bigger capital buffers.
It also called for measures to help wind down failing banks and the so-called ring-fencing of UK retail banking operations from other activities, including investment banking.
Bank of England Governor Mark Carney said last year that UK banks had raised over £130bn in capital since the crisis.
Sir John, a former Bank of England chief economist, told BBC Newsnight: "I think we've done some good building but there's an opportunity there to go a lot further, which should be taken. But the current policy stance is, no we don't need to."
He added: "I think the arguments, the evidence, the costs and the benefits, points to a need to go quite a bit further. I'd say we're roughly - global level - halfway of where we ought to be."
Capital from shareholders is a bank's first line of defence against an economic downturn or other shocks likely to cause losses.
The Bank of England earlier this year told banks to hold more capital, a way of reining in lending, as it expressed concerns about the fast growth of consumer credit like car loans and credit cards.
Regulators have forced banks to hold more, higher quality capital since the crisis, with enhanced standards required of the biggest institutions.
They have also increased supervision of the banking system, with regular stress tests, and introduced new ways to help resolve failing lenders without the need for a publicly-funded bailout.
Sir John also expressed his doubts over whether those tools could be relied upon in a serious crisis.
"I believe we'd be in huge trouble if a very large, very complicated banking institution got into trouble... there are tools, these so-called resolution tools, which didn't exist 10 years ago, which I think shift the odds in a slightly more favourable place," Sir John said.
"But I certainly wouldn't bet on those working perfectly. And I worry that the Bank of England and regulators internationally are placing huge reliance on these new, untried, untested tools working. That is a huge assumption to make," he added.
The Bank of England was approached for comment.
Others echoed Sir John's concerns.
Sheila Bair, a former chair of the US Federal Deposit Insurance Corporation, which promotes public confidence in the financial system, told Newsnight she still believed debt levels were too high in the banking system.
"The thing that scares me the most is people talking about loosening those capital standards. In my view, if anything, they need to be strengthened for the largest, most complex institutions. I think that would be very dangerous," said Ms Blair, who chaired the body from 2006 to 2011.
President Donald Trump has promised to scale back parts of the banking regulation passed in the US after the crisis. The rules, known as Dodd-Frank, have been dubbed a "disaster" by Mr Trump.
Ms Bair said she believed smaller, community banks had a "legitimate beef" about the costs and complexity of banking regulation and were caught up in some reforms that they should not have been.
But she said overall that moves to relax post-crisis reform were a worry, calling the effort "short-sighted".
"It could make the system less resilient, more unstable, really won't help the economy, it will benefit private shareholders, and that's not a good reason to do it," she added.