“People tend to get separated from their accounts when they move house and forget to update their address with all the companies they hold money with.
“It’s not just Premium Bonds, vast sums of savings, investments and pensions go astray this way too.
“It’s why in the first few weeks after you have moved in, it’s essential to make a list of everything you hold, and methodically work your way through contacting every company to let them know your new address.”
She also said people should consider consolidating accounts to make it easier to keep track.
The expert added: “The easiest way to avoid losing track of Premium Bond prizes is to have your winnings automatically paid into your bank account – or automatically used to buy more bonds. Around nine in ten prizes are paid this way.”
In addition to the 11 unclaimed prizes of £100,000, there are also 19 prizes of £50,000, 38 of £25,000 and 75 of £10,000.
The NS&I has no time limit on claiming prizes or bonds and people can find out whether they are a winner in different ways.
If you know your bond numbers, you can check on the NS&I website.
However, if you aren’t sure of your numbers, you can use NS&I’s tracing service by filling out a form from its website. There is also a ‘My Lost Account’ service.
The NS&I recently announced it had helped people find over £166 million in more than 52,000 accounts through its tracing service and My Lost Account.