The state pension age for women is gradually rising from 60 in 2010 to 65 in 2018.
The move has reduced the number of women the government pays a pension to by 1.1 million.
Raising the pension age for women will save the government 5.1 billion pounds each year.
The IFS found women aged between 60 and 62 are now £32 worse off a week.
They also found that levels of absolute poverty for women of that age group have risen to 21%.
Campaign groups say the government failed to adequately inform women of the change.
By 2020 the state pension age for men and women will rise again to 66.
Spending on pensions accounts for 42% of the total welfare budget in Britain.