Changes to the state pension age have unlawfully discriminated against women born in the 1950s, the High Court has heard.
Nearly four million women have been affected by the changes, which have raised the state pension age from 60 to 66.
Two claimants - Julie Delve, 61, and Karen Glynn, 62 - have now taken the Department for Work and Pensions to court, arguing that raising their pension age "unlawfully discriminated against them on the grounds of age, sex, and age and sex combined".
They also claim that they were not given adequate notice in order to be able to adjust.
The state pension age has been increased by successive governments in an attempt to ensure "pension age equalisation", so that women's state pension age matched that of men.
But, on the first day of the landmark legal case on Wednesday (June 5), Michael Mansfield QC said: "Although the object of the exercise was intended to be equalisation of treatment, in fact what has happened is the reverse."
Addressing a court packed with supporters from the campaign group BackTo60, Mr Mansfield said that "roughly 3.8 million" women had been affected by the changes.
The barrister said raising the state pension age discriminated against women born after 1950 on the grounds of their age, and also put women "at a particular disadvantage to men".
He submitted that "the claimants and many other women born in the 1950s" were not told about the changes "until shortly before their expected state pension age at 60", which caused "significant detriments" to many of them.
Mr Mansfield said many of the women expected to reach state pension age at 60 because that was "the practice and legislation in place for most of their working lives".
He added that women born in the 1950s had already suffered "considerable inequalities in the workplace", which he said were the result of "historical factors and social expectations".
Sir James Eadie QC, representing the DWP, argued that the changes were intended "to equalise the state pension age between the sexes" and "to ensure intergenerational fairness as between those in receipt of state pensions and the younger taxpayers funding them".
He added that the aim of raising the pension age to 66 was also to "make pensions affordable ... and to control government expenditure at a time of great pressure on public finances".
Sir James submitted that the government took "extensive" steps to notify women of the change to their state pension age, and added that "personal notification would have been very difficult if not impossible prior to 2003".
He concluded that the changes "pursue obviously legitimate aims", and that they "strike a balance between the public interest and the claimants' interests".
Speaking outside the Royal Courts of Justice in central London before the hearing, 61-year-old Elaine Hague, from Stoke-on-Trent, said she was "absolutely disgusted with the government".
She said: "I've worked all my life, since I was 16, and I feel totally humiliated and degraded by the government.