Mothers may have been underpaid about £1bn in state pension owing to information missing from their national insurance (NI) records.
Since 1978, the system has included credits for parents who took time out of employment to bring up children.
Until 2010, this was known as home responsibilities protection (HRP), but it may be missing from some NI records.
People who may be affected will receive letters from the government as part of a campaign to correct the issue.
Some people affected may have died and their families will be entitled to check their eligibility and make a claim for any arrears.