Women wishing to top up their basic state pension should do so now before a 50 per cent hike in buying back National Insurance (NI) contributions comes into force in April, experts have warned.

To qualify for a full basic state pension, currently £90.70 per week, women need 39 years of National Insurance contributions – although this is changing to 30 years from 2010.

Those who stay at home to look after children or relatives, and therefore do not make National Insurance contributions, can “buy back” years in order to qualify for a full pension.

However, hidden in the small print of the Pre-Budget Report on Monday was a rise in the cost of buying missing years – from £421 a year to £626 a year.

Men and women can currently buy back six years - however, the Pensions Act, passed on Wednesday night, states that this will increase to 12 years from 2010. The Government says that this will help provide those who stopped working with a better retirement - but campaigners say women are still getting an unfair deal.

Dot Gibson of the National Pensioners Convention said: “Up to 5 million existing women pensioners already fail to get a full state pension because they spent years raising children, caring for relatives or working part-time. Many of these women are now living in poverty and would have found it hard enough to buy back under the old rules, let alone now the cost has risen.

“The government has conned millions of older women by telling them they can buy back their lost pension, but failing to tell them exactly how much it is going to cost. Before this increase, only 1 in 10 were expected to take up the offer, but now it looks like the whole thing is unravelling. Women have a right to feel cheated.”

But a spokesman for the Department of Work and Pensions said: "We have always said the price of contributions will go up to reflect the increasing value of voluntary years under pension reform and reducing the number of required years for a full state pension to 30.

“This is a fair deal, especially for those women and carers who previously had been ineligible for the full state pension due to their working patterns. A large number will now be able to buy back six years extra National Insurance contributions to claim a full state pension and look forward to a more secure retirement.”