The billionth new £1 coin has passed through the Royal Mint's production line, as shoppers and retailers prepare to bid farewell to the old "round pound".
People are being urged to dig out their old pound coins from wallets, piggy banks and crevices in sofas before they lose their legal tender status on October 15. They can spend, bank or donate them to charity.
The new 12-sided pound coin, the shape of which resembles the old threepenny bit, entered circulation in March and boasts new high-tech security features to thwart counterfeiters.
By mid-July, there will be more new coins in circulation than old, the Treasury said. People have already returned 800 million of the old coins.
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 that time.
The production of the new coins follows concerns about round pounds being vulnerable to sophisticated counterfeiters. Around one in every 30 £1 coins in people's change in recent years has been fake.
The Treasury said research suggests one in three people still have old pound coins stashed away.
Exchequer Secretary to the Treasury Andrew Jones was visiting the Royal Mint's headquarters in Wales on Monday to officially mark the one billion milestone and strike the billionth coin with the Queen's head.
Mr Jones said: "This coin is the most secure of its kind in the world and was brought in to clamp down on the multimillion-pound cost of counterfeits.
"In less than 100 days, the round pound will lose its legal status. So people need to spend, bank or donate them by October 15."
Chief executive and deputy master of the Royal Mint, Adam Lawrence, said: " Many of the old round pounds returned will be melted down to make the new coins so we're asking everyone across the UK to make sure they check their coin jars and piggy banks for round pounds."