The law firm representing the Women Against State Pensions Inequality (Waspi) campaign has written a letter to the government laying out a list of demands to redress the movement's grievances.
The letter, which law firm Bindman's sent yesterday (8 March), warned if the government did not meet its demands the campaign would "consider all available legal and other options".
It comes five months after the campaign revealed it was planning legal action against the government.
A spokesman for Bindmans said the firm had written to Damian Green, secretary of state for work and pensions, to "seek redress for the injustice suffered by women born in the 1950s resulting from the substantial changes to state pension arrangements that were not properly communicated."
The letter put forward "a number of proposals" seeking to mitigate the impact of those changes.
Bindmans partner Jamie Potter said: "For too long the women of Waspi have patiently and courteously sought to engage with the government to obtain redress for the very serious injustices brought about by successive governments' failure properly to communicate the changes to the state pension age.
"Those calls have, unfortunately, fallen upon deaf ears. Our clients have therefore been forced to send formal legal correspondence to the government to call for concrete action.
"If the government refuses to consider these proposals, our clients will have no alternative but to consider all available legal and other options."
The letter was sent on International Women's Day, and coincided with a major Waspi demonstration outside parliament.
It came the same week government stated it would not allow women affected by the changes to access a reduced state pension early.
The government's statement was in response to a parliamentary petition started by Waspi splinter group, Waspi Voice.
In the statement, the government argued working longer was good for physical and mental health.