Organisations representing employers, consumers, workers and the elderly have joined the National Association of Pension Funds (NAPF) in urging the government to rethink its commitment to pot follows member.
In a letter in today's Financial Times, the unlikely coalition said the government's preferred option for consolidating small pots had a " a number of inherent risks and weaknesses".
The signatories said pot follows member could expose people to repeated transaction costs as assets had to be divested and reinvested with every transfer.
They added members risked being transferred from well-run arrangements to poor quality schemes and said the system could push savers into investments that had high liquidity but limited growth.
They said: "The regular movement of billions of pounds of assets might itself have an impact on the market. In addition, the burden of establishing and administering a system to enable these transfers could impose significant costs on pension schemes and employers."
The letter was signed by the leaders of Age UK, manufacturers organisation EEF, the Trades Union Congress and Which?, as well as NAPF chief executive Joanne Segars (pictured).
Today's letter urged the government to accept amendments to the Pensions Bill tabled by Labour that would give more flexibility when developing a system to consolidate small pots.
The majority of respondents to a government consultation in 2012 supported a system that transferred small pots to large scale aggregators ahead of the pot follows member system.
But the signatories cautioned that alternative proposals also required more detailed thinking to ensure the interests of savers were safeguarded.
The letter concluded: "We believe that widening the scope of the legislation to allow the development of more than one potential model would be the most useful step the government could take to ensure we can tackle the proliferation of small pots effectively."
The proposed amendments will be debated in the House of Lords today.
CLICK HERE for analysis of whether the plan would solve the 'small pot problem'.
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