More than two million women are not building up any entitlement to the basic state pension, according to the Department for Work and Pensions.
Many women miss out on a full state pension because they earn too little or take time off work to look after children or relatives, it said.
In addition, fewer women are contributing to the basic state pension and as a result have to claim the pension credit, a means-tested benefit.
On top of that, fewer women than men contribute to a private pension. In total, 38% of working age women are paying into a private pension – compared to 46% of men.
Pensions minister Stephen Timms admitted that the current state pension system is unfair to women.
“The report does show that the National Insurance system hasn’t been delivering fair outcomes up to now,” he told BBC Radio 4. “Thirty per cent of women reach state pension age at the moment on a full basic state pension, compared to more than 80% of men.”