The round £1 will be legal tender alongside the new, more-secure coin until 15 October.
The public are being urged to use their current £1 coins or bank them before they lose their legal tender status.
The government estimates around a third of the £1.3 billion worth of coins stored in piggy banks or saving jars around the UK are the current £1 style.
Some of those returned by the public will be melted down and used to make the 12-sided version.
The new style was announced in the 2014 budget and has been billed by the Royal Mint as "the most secure coin in the world".
Its introduction will come as a new set of coin designs are also brought into circulation, celebrating the achievements of Jane Austen and Sir Isaac Newton.
The Royal Mint said the new designs have a "strong pioneering theme" and will start appearing this spring.
A Jane Austen £2 coin will celebrate the author 200 years after her death, while another £2 version will remember the Royal Flying Corps.
A 50p coin will mark the achievements of mathematician Sir Isaac Newton, a one-time Master of the Royal Mint.
Dr Kevin Clancy, director of the Royal Mint Museum, said: "This is a particularly significant year in Royal Mint history as we welcome in the new 12-sided £1 coin, with its innovative security features.
"This year we also mark the achievements of Jane Austen, Sir Isaac Newton and the Royal Flying Corps - all pioneers in their own field."

Julia Knight, 56, had denied trying to kill Irene Robson, 81, but admitted administering the insulin at Horton Hospital in Banbury in October 2014.
During a trial at Oxford Crown Court, Knight said she did it so her mother would not be discharged from hospital.
Prosecutors said it was a "premeditated, cold-blooded attack".
The trial heard Knight forged a prescription for insulin, which she collected at a Sainsbury's store in Banbury.
Knight told jurors she had wanted to make her mother a "little bit poorly" and described it as "a stupid, stupid, thing to do".
Updates on this story and more from Oxfordshire
She also told the court she had attempted suicide in 2006 by injecting herself with insulin.
Det Insp Craig Kirby of Thames Valley Police said: "This is an incredibly sad case.
"It was only following a large scale police investigation, when the weight of evidence became overwhelming, that Julia Knight finally admitted injecting her mother with insulin, something she had repeatedly denied during a number of police interviews.
Adrian Foster, chief crown prosecutor for Thames and Chiltern Crown Prosecution Service said: "It is impossible to imagine a more vulnerable victim.
"Knight embarked on a premeditated, cold-blooded attack on her mother by illicitly injecting her with a potentially lethal dose of insulin while she was an inpatient on a ward after suffering a fall at home.
"The insulin, obtained by Knight via a forged prescription, caused Irene's blood sugar concentration to drop to a lethal level and she fell into unconsciousness, but hospital staff managed to save her life.
"There is no doubt that as a qualified practising nurse and with her medical knowledge of the effects of unprescribed insulin that Knight intended to kill her mother."
Ms Robson died in September but the trial heard her death was not relevant to the case.
Knight is due to be sentenced on Friday.

The US network said it was "not an easy decision" and had "enormous respect" for its creators and cast.
Set in the 1970s, the series told of a charismatic record executive trying to revive his struggling label.
Its ensemble cast included Bobby Cannavale in the lead role, Olivia Wilde as his wife and Sir Mick's son James as a rebellious British singer.
Sir Mick and Scorsese were among the show's executive producers, with the latter directing its pilot episode.
The series premiere in February was watched by 764,000 viewers, though the average audience subsequently dipped to 650,000.
"After careful consideration, we have decided not to proceed with a second season of Vinyl," HBO said in a statement.
"We have enormous respect for the creative team and cast for their hard work and passion on this project."
Ray Romano, another actor in the cast, told an Australian newspaper on Thursday the cancellation felt like "a knife in the guts".
Wilde, meanwhile, thanked her Twitter followers for their "kind words" and said she had been part of "something special".
The new 12-sided £1 coin will enter circulation on 28 March, the government has said.
[NEXT_CONCEPT]
A nurse who injected her mother with insulin as she lay in hospital after a fall has been found guilty of attempted murder.
[NEXT_CONCEPT]
Sir Mick Jagger and Martin Scorsese's music business-based drama Vinyl has been cancelled by HBO after one season.