They amount to £80.3 million in prize money and date back as far as the Sixties.
NS&I hands out thousands of prizes every month ranging from £25 to £1 million.
Although bondholders are usually contacted when they win, many people forget to change their details when they move house or switch personal details. A letter can get lost in the post or emails end up in junk folders. They then may also forget to check their numbers every month, which can be done online or on the NS&I app.
Some people may have forgotten about their bonds entirely, with one unclaimed prize of £25 in Hampshire and the Isle of Wight dating back to August 1963.
That £25 would now be worth £694 in today's money, according to this inflation calculator.
The region is where one of this month's £1 million prize winners is registered. But there are 50,694 other unclaimed prizes there totalling £1,528,175.
The largest unclaimed prize in Hampshire and the Isle of Wight is £5,000 from the January 2019 draw.
In Norwich, where September's second £1m winner is from, there are 8,749 unclaimed prizes worth £260,700. The oldest is a £25 from May 1969, which would today be worth £542.
NimbleFins previously reported NS&I has raised the chances of winning a Premium Bond prize to its highest level in 15 years. (Read here if they are worth buying)
But the investment is not worth it if you forget about them.
How to check if you've won Premium Bond prize
The good news is there is no deadline to claim a Premium Bond prize and NS&I keep hold of the cash until it’s claimed.
As of September 2023, there were 2,300,822 unclaimed Premium Bond prizes worth £80,254,250.
That's up by about 30,000 (and £800,000) since just November, meaning thousands of prizes are going unclaimed every month.
The best way to avoid missing out on a prize is to have winnings automatically paid into your bank account or reinvested in more bonds, NS&I says.
You can do this online.
But if you want to check if you have unclaimed winnings, the quickest way to do this is to use the NS&I prize checker online or on its app.
You can also write to NS&I to ask for your prize history. You'll need to tell them your name, address, date of birth, NS&I number or Premium Bonds holder's number. The address is: NS&I, Sunderland SR43 2SB. Note, its Blackpool and Glasgow addresses are no longer in use.
For more information on contacting the NS&I by post, and what to include in the letter, click here.
You can change your details online, by phone, or by post. For more information click here.