The Government says it intends to press ahead with its timetable to raise the state pension age for women despite the threat of a revolt by Conservative and Liberal Democrat backbenchers.

MPs of all parties have urged ministers to rethink plans to speed up the equalisation of the pension age for women and men amid warnings hundreds of thousands of women have not had time to plan properly for their retirement.

However, the Department for Work and Pensions (DWP) insists the changes will go ahead as planned, warning any delay would cost the taxpayer £10 billion.

Under the Pensions Bill, which receives its Commons second reading today, the state pension age for women will go up from 60 to 65 in 2018, two years earlier than planned under Labour, and rising to 66 in 2020.

The move has provoked intense cross-party opposition, with MPs warning it discriminates unfairly against women in their late 50s, who will now have to wait longer than expected to receive their pension.

Lorely Burt, the chair of the Lib Dem parliamentary party, says up to 500,000 women will be hit as they "won't have time to plan their retirement and many will be financially a great deal worse off".

For Labour, shadow pensions minister Rachel Reeves says ministers should bow to public pressure and rethink their proposals.

"It is simply wrong to punish women by moving the goal posts at this late stage," she says.

"There is strong and vocal opposition to these unfair pension changes across the UK. It's not too late for David Cameron to think again."

Ros Altmann, director general of Saga and a former government adviser on pensions, warns ministers could face a costly legal challenge if they do not change course.

"Ministers must listen to reason on this issue. The current plans are unfair and may, indeed, be illegal in public law terms, since they clearly do not give women adequate notice of the large changes in pension age that they face," she says.

"A legal challenge to the fairness of these proposals is likely to be difficult for the Government to defend and could end up costing the taxpayer significant sums in court fees and compensation for those affected."