More than 400,000 women on the verge of retirement will miss out under the Government's pensions reforms while men in the same position will still benefit, Labour has said.
Ministers had insisted their White Paper would help women by handing them an improved state pension.
But shadow pensions minister Greg McClymont said that 429,000 women born between April 6, 1952, and July 6, 1953, will miss out on the new single tier pension, while men would controversially still be eligible.
In a question to Pensions Minister Steve Webb in the Commons, he said: "Even at my speed reading of today's White Paper there will be heavy losers, steep cliff ledges and significant costs if this proposal goes ahead.
"For example, the briefing from the Government over the weekend was at pains to emphasise the women-friendly aspect of these measures. But can I ask you directly about the 429,000 born between 6 April 1952 and 6 July 1953? Is it the case that these 429,000 women will not qualify for the single tier state pension, yet men born (between) the same dates will do so? Is that the case?"
Mr McClymont also claimed existing pensioners would miss out. He added: "And let me dig a little deeper? You referred to existing pensioners - is it the case that this proposal excludes all those existing pensioners and all those who intend to retire before 2017?
"If that is the case, what is your message to those 15 or 16 million pensioners in my calculation who will not be eligible for the new pension?"
Mr McClymont also accused the Government of a "tax grab" by handing back money from the 1.4% National Insurance rise to the Treasury. He added: "As I understand it, the money raised from this tax hike will not be re-invested in to the new State Pensions but will fall straight in to Treasury coffers. Is that indeed the case? How much money does this tax grab raise? Why is this money not being re-invested in to the new state pension?"
In reply, Mr Webb said the current system penalised many women in their late 50s for staying at home and looking after their children. He added: "Although they got some protection on the basic pension, they didn't get protection on SERPS (State Earnings Related Pension Scheme) and the second state pension. We are putting that right for that very group of women.
"You asked about women born in certain months and the equivalent men. The changes are based on state pension age and you know the state pension age is different for men and women and so of course the implications are different for men and women."