Thousands of married women who reached state pension age before 6 April 2016 have been underpaid their state pension for more than two decades.

Documents revealed in the budget last month highlighted that approximately 200,000 pensioners have potentially been underpaid up to £3bn between them in state pension entitlement, with individuals entitled to an average pay out of £13,500 to make up the shortfall.

The blunder is thanks to the extreme complexity of the state pension system, which has been tweaked and reworked by many governments over time, making it nigh-on impossible to know how the details work for everyone.