In a letter to MPs, the Association of Independent Financial Advisers (Aifa) described its concerns over some aspects of the government's proposed personal accounts scheme.
Chris Cummings, director general of the association, suggested that the pensions bill, which is set to enrol people into the personal accounts scheme, is flawed.
"Because of means-testing, up to one in five future pensioners could effectively be worse off than those who opt-out of personal accounts and choose to rely on state benefits," he stated.
Additionally, Mr Cummings raised concern about the impact legislation will have on existing pension schemes that contribute more than the amount required under the new scheme.
The expert also claimed that there is a lack of advice regarding the introduction of personal accounts for both employers and employees - a fact he said would "further exacerbate this problem".
Meanwhile, Ian Naismith, head of pensions market development at Scottish Widows, urged the government to "remain committed" to the proposals it originally announced.
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