Frances Kerr spent six months fighting for about £2,400 a year to be added to her state pension.

Kerr, 66, started getting the new state pension in December and gets the full amount of £221.20 a week. But because her husband died in 2011 aged 72, she knew she might be entitled to inherit part of his state pension too.

“My mum started got some extra money after my dad died and my sister did after her husband died, so I thought I might be entitled to something extra too,” Kerr said. “But the system is awful to deal with. I would phone up and get passed between different departments. It felt like I was going round in circles.”

Kerr, a former carer from Harrow in northwest London, contacted the Department for Work and Pensions (DWP) in January and was told to send a copy of her marriage certificate. It took six weeks for this to be uploaded to the DWP system. In March was told she was not entitled to inherit any of her husband’s state pension.

But Kerr suspected this was wrong. She contacted Steve Webb, a former pensions minister and a partner at the consultancy Lane, Clark and Peacock (LCP), who spoke to the DWP on her behalf. In June she was told her state pension payments would go up to £268 a week — £47of which had been inherited from her husband, Nathaniel.

“I’m glad I’m getting the extra money, but it was so confusing. They said I wasn’t entitled to inherit anything and then suddenly I was. I still don’t know how they worked it out,” Kerr said.

Thousands of people who have lost a spouse could be in the same position, according to LCP. About 68,000 widows and widowers start getting the state pension each year and Webb said the DWP should investigate how many are being underpaid. “I’ve seen multiple cases in the past few months where the DWP has failed to automatically add any inherited state pension,” he said.

Hundreds of thousands of people are thought to have been underpaid their state pension. The DWP estimates that 210,000 people were underpaid by £1.3 billion because of gaps in their national insurance record from time spent caring or looking after children. An additional 165,000 people were underpaid by about £1.2 billion because of DWP errors.

Married women, widows and those over the age of 80 who get the old state pension are most likely to be affected. The DWP is in the process of identifying people who were underpaid and most should get what they are owed automatically. About £572 million was paid out between January 2021 and February 2024.

You may be able to inherit some of your spouse or civil partner’s state pension — even if they died before they started to claim it. If they reached state pension age before April 2016, when the new state pension was introduced, or would have done so had they lived longer, then you can normally inherit some of their additional state pension. You may also qualify if your partner died before April 2016 even if they would have reached state pension aged later.

The old state pension was made up of two parts. The basic state pension, which pays up to £169.50 a week for the 2024-25 tax year, and the additional state pension, which is based on your earnings and national insurance contributions.

You can inherit some or all of this additional part — the amount you can get will depend on when your partner died and their date of birth.

• Am I entitled to WASPI women compensation?

If your partner died before October 6, 2002, you can inherit all of their additional state pension. If they died on or after this date you can still get all of it if they were a man born on October 5, 1937, or before, or a woman born on October 5, 1942, or before. If they were born later you get less — with the amount falling gradually to 50 per cent if they were a man born after October 6, 1945, or woman born after July 6, 1950.

If your partner reached state pension age after April 2016 you may still be able to inherit part of their pension. People who would have been entitled to a large additional state pension under the old pension system can get a “protected payment”, which is paid on top of their new state pension. You can inherit half of a protected payment.

You cannot normally inherit your late partner’s state pension if you remarry or form a new civil partnership before you reach state pension age.

You can call the Pensions Service to check whether you are entitled to an inherited pension on 0800 731 0469. LCP has created an online tool at lcp.com to help you check if you are due to inherit anything from a deceased partner’s state pension.

• How much state pension will I get?

The DWP said: We want to ensure pensioners receive all the support to which they are entitled and we have a tool to help them understand what state pension they can inherit. Delays can occur to a customer’s state pension award when not all the information we need is provided. In these cases, we will make an award based on the customer’s own national insurance record until we have the required information. Once we have the necessary documentation, we will revise the customer’s claim as soon as possible.”

The DWP’s tool for assessing your eligibility to inherit a state pension is at gov.uk/state-pension-through-partner