Unite's assistant general secretary, Gail Cartmail, said this could only be true if someone, such as a nurse, worked for an extra eight years to 68, after 43 years in the career average scheme, compared to someone retiring normally at 60 after just 35 years in the current final-salary version.
"Danny Alexander is making extraordinary claims in order to mislead and manipulate the public about the government's pensions proposals," she said. "He's using distorted figures to conceal the way in which government proposals will reduce pensions".
Unite said its own calculations showed that a nurse who retired at 60 in the new career average scheme would have a pension 40% lower than a counterpart with an equivalent career but retiring at 60 in the final-salary scheme. And working another five years to 65 would still leave them 20% worse off.
The union also accused the minister of glossing over the increased pension contributions that staff are being asked to make in the next few years, which it said would amount to some staff paying 50% more each year.