Phil Loney, group chief executive of Royal London, said the changes had been carried out on "entirely unrealistic timescale" which could leave retirees exposed and failing to get the best outcome.
He claimed the Government had just "thrown into the place" the new rules and failed to take a "meticulous approach to implementing change".
He said: "We wholeheartedly support the policy objective but customers are not ready for the new pension freedoms, which have been thrown into place in an entirely unrealistic timescale.
"I fear that many will make the wrong, often irrecoverable decisions about their retirement and this will result in some very poor outcomes. The simple fact is that many people, perhaps most, have not engaged with pension freedom and lack the basic financial knowledge to take the next steps.
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"George Osborne's pension reforms have the potential to become famous for helping people to improve their retirement incomes but without plentiful and affordable financial advice they risk becoming an infamous example of political bungling."
He suggested the new guidance service Pension Wise would be inadequate.
Despite referencing the help offered by The Pensions Advisory Service in its wake up packs sent during the last three months of 2014 less than 2% of Royal London customers without advisers made contact with TPAS - 71 out of 3,600 letters.
Mr Loney said: "I remain to be convinced that a new leaflet with a new logo, and a publicity campaign will dramatically improve response rates anytime soon.
"Meanwhile Citizens Advice report that they will only have a very limited capacity for face to face guidance in place for April."
He added: "Given the importance of getting sound guidance at retirement, we are working with the Government's Pension Wise team on different signposting techniques that we hope will be more effective."
The Treasury responded with this statement: "The government believes that people who have worked hard and saved all their lives should have the freedom to decide how to use their savings and the guidance to help them make good decisions.
"People will have access to a new guidance service that will be delivered to rigorous FCA standards, and empowers and educates people so they can make their own, informed choices and confidently engage with the pensions industry."
Meanwhile, Royal London announced new life and pensions business sales for the year ending 2014 (on a PVNBP basis) were up 39% at £4,826m (£3,463m in 2013). Individual pensions' new business figures were up 25% on last year. In the fourth quarter sales rose 65% compared to Q4 2013 (£443.2m Q4 2014 and £269.5m in Q4 2013).