Campaigners have threatened the government with legal action unless it reconsiders its decision to rule out spending £10bn compensating “Waspi” (Women Against State Pension Inequality) women hit by the change in the state pension age.

Last March, the parliamentary and health service ombudsman recommended compensation should be paid to women born in the 1950s whose pensionable age was gradually raised to be equal with men, arguing the changes had not been properly communicated to them.

In December, the government announced there would be no compensation for this group. While Keir Starmer acknowledged concerns raised by parliamentarians over the government’s decision, he said at the time: “Ninety per cent of those impacted did know about the change – and in those circumstances, the taxpayer simply can’t afford the burden of tens of billions of pounds of compensation.”

The Waspi campaign has sent a “letter before action” to the Department for Work and Pensions (DWP), warning of high court proceedings if the issue is not resolved.

Angela Madden, the chair of the group, said members would not allow the DWP’s “gaslighting” of Waspi women to go unchallenged.

She wrote: “The government has accepted that 1950s-born women are victims of maladministration, but it now says none of us suffered any injustice. We believe this is not only an outrage but legally wrong.

“We have been successful before and we are confident we will be again. But what would be better for everyone is if the secretary of state now saw sense and came to the table to sort out a compensation package.

“The alternative is continued defence of the indefensible but this time in front of a judge.”

The chancellor, Rachel Reeves, and the work and pensions secretary, Liz Kendall, were among senior ministers who backed the Waspi campaign when Labour was in opposition.

Approximately 3.6 million women in the UK were affected by the phased changes to bring their retirement age into line with men, first announced in 1995. In 2011, the coalition government sped up the equalisation process.

Campaigners have claimed women suffered financial hardship and have had to rethink their retirement plans.

Brian Leishman was one of 10 Labour MPs to support a bill introduced by the SNP urging the government to publish measures to address the findings of the ombudsman report. Leishman said the next UK government could be a “hardline far-right effort” if Labour did not deliver “improved living standards”.

A government spokesperson said: “We accept the ombudsman’s finding of maladministration and have apologised for there being a 28-month delay in writing to 1950s-born women.

“However, evidence showed only one in four people remember reading and receiving letters that they weren’t expecting and that by 2006 90% of 1950s-born women knew that the state pension age was changing.

“Earlier letters wouldn’t have affected this. For these and other reasons the government cannot justify paying for a £10.5bn compensation scheme at the expense of the taxpayer.”