The Royal Mint has produced 1.5 billion of the new coins which will be distributed to banks and retailers over the next six months.
Around four million new pound coins are being produced each day and people can now expect to find them in their change in the coming days and weeks.
The £1 coin has been dubbed “the most secure coin in the world” and boasts a number features designed to make it more difficult to counterfeit.
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This includes a hidden high security feature built into the coin as well as micro-sized lettering inside both rims.
There is also a hologram-like image that changes from a ‘£’ symbol to the number ‘1’ when the coin is seen from different angles.
Adam Lawrence, chief executive of Royal Mint, said: “It’s been designed to be fit for the future, using security features that aim to safeguard our currency, and currencies around the world, for years to come.
“Staying ahead of sophisticated counterfeiters remains a constant challenge and this coin helps in that battle.”
Round £1 coins will now start to be returned to The Royal Mint - some of which will be reused to make the new coin.
Although many businesses will have upgraded their machines to accept the new coin from March 28, not all machines will accept the new coin from the date of introduction.
The old coin will remain legal tender until October 15 this year, after which shops do not have to accept it. But, people will still be able to return old coins to the bank after this time.
Baroness Neville-Rolfe, Commercial Secretary to the Treasury, said: “This is a giant step forward to help stamp out counterfeit coins and save businesses and the taxpayer millions of pounds every year.
“Today marks the start of the six-month transition period, so I’d urge everyone to make sure they spend, return or donate their old round pounds before October 15.
“We have been working hard with businesses over the last three years to help make this changeover as smooth as possible.”