The all-round British pound is set to lose its status as legal tender on 15 October, but experts estimate that there are roughly 400m of the coins still in circulation that will soon become worthless to shops and retailers across the country.
The hunt is now underway for consumers who still have what amounts of millions-worth of the dying currency tucked away.
UK ministers have said that roughly £1.7bn of the old coins have already been withdrawn from circulation - but there is still more work to be done.
From 16 October, unspent coins will become impossible to use and will have to be exchanged at banks.
The Royal Mint said: "We would encourage you to spend, bank or donate your round £1 coins before October 15. We have urged businesses and their frontline staff to, where possible, prioritise the new coin when giving customers their change.
"Customers are entitled to ask for their change in any way they wish, but until 15 October businesses can continue to give out the old coin."
The new 12-sided £1 coin is being touted as one of the most secure currencies in the world and first became legal tender in the UK earlier this year, on 27 March 2017.
The new round £1 coin had to be replaced for the first time in more than thirty years because of its vulnerability to counterfeiters. Experts believe that approximately one in thirty round £1 coins in circulation on high streets is actually a counterfeit.
The new security features on the updated pound coin include a hologram-like image that changes from a '£' symbol to the number '1' when the coin is seen from different angles.
It also has micro-lettering and milled edges, the government has said.
The new coins are being produced by The Royal Mint, based in South Wales, at a rate of up to four million per day according to state statistics.
Both the Royal Mint and HM Treasury have reportedly been working with British businesses for the past three years to help them prepare for the imminent change.
On 20 September, Children in Need, The Royal Mint and HM Treasury kicked off a nationwide campaign to help locate the missing millions, urging those who find them to donate some to charity.