The survey by retirement specialist Just Group found many over-55s, potentially millions of people, ignorant about what the State Pension would pay them.

Many also overestimate how much they will receive at normal State Pension Age which will be 67 from 2026 and is currently 66.

The research of more than 1,000 retired and semi-retired people aged over 55 found that four in 10 (38%) had not checked their State Pension forecast.

This rose to 40% of those who had not reached State Pension Age and 46% for those who claimed to have retired earlier than expected.

Just Group said the overall figures suggested more than a third of retirees did not check their State Pension forecast before they retired despite 1.2m households relying on the State Pension as their “primary source” of income in retirement. 

Among the two-thirds of retirees who checked their forecast State Pension income, nearly a fifth (17%) said that it was – at a minimum – £250 less per year than they were expecting. In contrast, one-in-ten (9%) retirees said that it was at least £250 more per year than they were expecting.

Stephen Lowe, group communications director at Just Group, said: “It’s easy to see why people may assume they’ll simply get the full State Pension, but for many people this won’t be the case. The last thing these households need when they come to retire is the nasty surprise that their State Pension is less than they thought.

“The government offers a State Pension forecast service and we urge anyone approaching retirement to use it – ideally in advance of beginning to retire.”

• Editor's Note: SPA clarified.