Round 1 pound coin will become worthless after October 15 as the government introduces a 12-side coin.
British people are hoarding around 433 million pounds in the old style coins in piggy banks, car ashtrays and down the back of sofas and armchairs, experts say as the Treasury prepares to phase them out this year.
The old coin has been a favourite of savers since it was introduced in 1983.
But it is set to be replaced by the new 12-sided 1 pound coin in March and the round pound will cease to be legal tender by October 15.
The Sun reports that the government estimates that 1.3 billion pounds worth of coins are stashed away around a third - or 433 million pounds - of which are pound coins.
On Sunday the final design for the 12-sided coin was unveiled and ministers warned the old one would become worthless, it said.
The new design is made of two metals, with a gold- coloured outer ring and a silver-coloured inner ring.
It has the pound symbol that changes to the number '1' when the coin is seen from different angles and is thinner and lighter than the current coin.
The currency change was announced in the 2014 Budget.
Chief Secretary to the Treasury David Gauke said: "March 28 should be an important date in everybody's calendar this year - as we will have a new quid on the block. This is a historic moment as it's the first time we've introduced a new 1 pound coin since 1983, and this one will be harder to counterfeit than ever before. Our message is clear: if you have a round one pound coin sitting at home or in your wallet, you need to spend it or return it to your bank before October 15."
After the introduction in March there will be a transition period when vending machines may only accept old coins.
Helen Dickinson, CEO of the British Retail Consortium (BRC), said: "Through the British Retail Consortium, most major UK retailers have been engaged in discussions and planning with the Royal Mint and Her Majesty's Treasury since the March 2014 announcement that a new pound coin would be introduced in 2017."