By Sonia Elks

LONDON, Nov 6 (Thomson Reuters Foundation) - Millions of British women feel "betrayed" and many are facing poverty, campaigners said on Tuesday, as the age they can claim the state pension rose to 65 to match that of men.

The government is accused of failing to give women enough warning of the steep rise in the state pension age, which experts say leaves women poorer than men because it amplifies the effect of lower lifetime earnings.

"For some women it's absolutely devastating," said Debbie de Spon, a spokeswoman for the campaign group Women Against State Pension Inequality (WASPI).

"We are angry that we have been treated this way having gone through our lives doing what was expected of us ... A word we often hear is 'betrayed'."

The government said means-tested support was available for those struggling to manage.

"The decision to equalise the state pension age between men and women was made more than 20 years ago and achieves a long-overdue move towards gender equality," said a spokesman for the Department for Work and Pensions.

In the past, British women were able to claim their pension at age 60, while men had to wait until they were 65, but the government has moved to gradually equalise their positions.

The overall pension age for both sexes is also due to rise to age 68 by the mid-2030s to offset longer lifespans.

A decision was first made in 1995 to lift the women's pension age by 2020, but WASPI says many women were not informed for more than a decade afterwards.

The government then decided to speed up the process in 2011.

The changes particularly affected about 3.8 million women born in the 1950s who were approaching their pension age as the changes took place, said WASPI.

Some will have work six years extra, it said.

It is calling for a lower bridging pension for women born in the 1950s during the extra years they have to work or compensation for those who have reached pension age but have suffered as a result of the way the changes were introduced.

Experts said the move will leave women worse off than men because they usually have less saved for retirement.

A study carried out by pensions and asset management firm Aegon last December found that at age 50 women have an average pension fund of 56,000 pounds ($73,220) - half that of men.

The firm's director Steven Cameron said it was "hard to argue against equalising state pension ages", but there was no doubt it left older women at a disadvantage to men.

"It has just made the gender pension gap overall bigger because up until now the one thing women had that men didn't have was an earlier state pension age," he told the Thomson Reuters Foundation. ($1 = 0.7648 pounds) (Reporting by Sonia Elks @soniaelks; Editing by Claire Cozens. Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's and LGBT+ rights, human trafficking, property rights, and climate change. Visit http://news.trust.org)