Londoners are missing out on more than �5m

National Savings and Investments said 400,000 people residing in the UK and abroad had failed to collect prizes.

The main reason why there were so many unclaimed, was because people forgot to change their address details.

Many bond-holders who had been given them as a child have simply forgotten them - one teenager from Essex is not aware they have won �10,000.

The Treasury-backed institution said it had tried to trace the missing winners, and was now trying to raise awareness through the media.

There is no time limit on claiming, and the oldest unclaimed prize date's back to the first draw in June 1957.

People living in London have the most unclaimed money at �5.5m, followed by Scotland at �1.6m and people in Lancashire at �1.1m.

Gill Cattanach, NS&I commercial director, said: "Our aim is to reunite Premium Bond customers with �23.6 million in unclaimed prizes and we urge those customers who've forgotten to tell us they've changed address to get in touch to check if they are a winner."