About £450 million has been "lost" overnight as old pound coins stopped being legal tender.
As of midnight on Sunday, hundreds of millions of round coins were replaced with the new 12-sided design launched in March.
Some stores, including Tesco, have promised to let people keep spending the old coin for a limited period, while banks and building societies also offered extra time to deposit.
It has now been estimated around 400 to 450 million old round coins were still languishing unspent in members of the public's purses, piggy banks, and sofa nooks.
People had been offloading their old coins at a rate of about 60 million a week in the lead up to Sunday night's deadline.
Some major stores have said they will give shoppers an extra window of opportunity to spend them.
Iceland and Poundland have said they will continue to accept the old-style coins until October 31, while Tesco has said it will continue accepting them for a week after yesterday's deadline.
Major banks and building societies have also said they will continue to accept deposits of the old round pound.
People can also deposit them into any of their usual high street banks or through the Post Office after this date.
Martin Kearsley, banking services director at the Post Office, said: "Thanks to an agreement with all UK high street banks, everyone can deposit old pound coins into their usual high street bank account at their local Post Office branch."
A spokeswoman for the Royal Mint said: "Following the end of legal tender status, the current round £1 coin can continue to be deposited into a customer’s account, either business or personal, at most High Street Banks including RBS, NatWest, Ulster, HSBC, Barclays, Lloyds, Santander, Nationwide, Clydesdale, Yorkshire Bank, Halifax, Bank of Scotland and The Post Office.
"Specific arrangements, including timings and deposit amounts may vary from bank to bank. It is recommended that members of the public consult with their bank directly."
One pound coins were first launched on April 21 1983 to replace £1 notes. The Royal Mint has produced more than two billion round pound coins since then.
The new 12-sided pound coin, which resembles the old threepenny bit, entered circulation in March and boasts new high-tech security features to thwart counterfeiters.
The production of the new coins follows concerns about round pounds being vulnerable to sophisticated counterfeiters. Around one in every 30 old-style pound coins in people's change in recent years has been fake.
Read More
MORE ABOUT