DB schemes carry significant risk to members, are working ineffectively, and their benefits to the wider economy have reduced.
Those are among findings from the Pensions and Lifetime Savings Association's (PLSA) Defined Benefit Taskforce in its interim report.
It said that some £81bn a year is paid out in salary-related pension benefits from almost 6,000 private sector defined benefit schemes.
Despite between them holding assets under management of £1.5tn, more than 5,000 of these schemes are in deficit.
Taskforce chair Ashok Gupta said: "The current state of DB poses a significant risk to members' benefits for all but the most strongly funded schemes.
"The Pension Protection Fund helps mitigate these risks and plays a vital role in providing security for scheme members whose schemes have failed, but members still bear the risk the PPF does not cover.
"For many members the risk is they will lose 15-20% of their benefits.
"And member awareness of this risk is extremely low."
The taskforce concluded that employers were "running to stand still".
In 2015 employers paid some £31bn into their DB schemes of which £11bn was deficit recovery contributions, almost as much as the entire UK foreign aid budget.
"The current system has built up over decades but is not fit for the future," Mr Gupta said, adding that the "current inefficiency of the DB sector" affected both scheme members and sponsors and also the wider economy since the money swallowed by deficit recovery could potentially have been put to other uses.
PLSA chief executive, Joanne Segars said: "The system we have is not working as well as it could, it is inflexible and costly.
"It only allows for binary outcomes of complete success or complete failure; greater flexibility in the system may help to create better outcomes for scheme members.
"The system is also fragmented and the potential exists for consolidation to deliver better value to scheme members and their sponsors."
She said the next phase of the Taskforce's work would be to collaborate across the pensions and investment sector with government, regulators, social partners and industry to develop recommendations to support the sustainability of DB.
* The current system is too fragmented. Scheme consolidation should be investigated.
* The regulatory approach to scheme resolution is inflexible. Work should be undertaken to deliver better solutions to scheme resolution and remove regulation little benefit.
* Benefit design and change is too rigid. A more flexible approach could be implemented.
* The current approach to pension scheme risk bearing is sub-optimal.
The Institute and Faculty of Actuaries (IFoA) Immediate Past President, Fiona Morrison said: "The IFoA welcomes the publication of the PLSA's report.
"As the report identifies, the reality is that many members remain unaware that the security of their benefits relies on the sponsoring employers solvency.
"Similarly, many sponsors remain unclear about the cost and risk associated with meeting their pension liabilities.
"We welcome any opportunity to develop new approaches in communicating these risks to employers, members and trustees."