Commemorative coins minted to mark Brexit on 31 October will be "recycled" after the UK's exit from the EU was delayed by a further three months.
The Royal Mint had been asked to produce new 50p pieces featuring the UK's scheduled departure date.
However, in a move first reported by Bloomberg, the coins are to be scrapped as the UK will not now leave the EU this week.
A Treasury spokesman said a coin will still be produced to mark Brexit.
He added that it "will enter circulation after we have left" the European Union.
According to the Royal Mint website, precious metals are recycled by being sorted and shredded before being melted down. The 50p coin is made from copper, increasingly considered a precious metal, and nickel.
Metals are then purified and solidified before being turned into new products, the site adds.
The UK was due to leave the EU at the end of the month, but the PM was required to request an extension after Parliament failed to agree a Brexit deal.
On Monday, Mr Johnson formally accepted the bloc's offer of an extension until 31 January 2020.
The PM's acceptance means that the UK will not now leave the EU on Thursday - despite a "do or die" promise he repeatedly made during this summer's Tory leadership campaign and since entering Downing Street in July.
Up to 10 million new coins were reportedly being made ready for the Halloween deadline, in a project championed by Chancellor Sajid Javid since he took office in July.
He had asked officials to look at whether it would be possible to produce the coins in time for the 31 October date. They were due to feature a message of "Peace, prosperity and friendship with all nations".
Mr Javid's predecessor, Philip Hammond, had planned a limited edition of about 10,000 commemorative 50p coins to be sold to collectors for £10 each.
Under plans signed off by the Queen earlier this month, three different types of 50p Brexit coins were to be made from gold, silver and cupro-nickel - a mixture of copper and nickel.
The silver and cupro-nickel pieces were set to become legal tender.
The design, featuring the monarch's head and a standard inscription on one side and the special message and date on the other, was approved at a meeting of the Privy Council on 8 October.
The cost of designing and producing commemorative coins is met by the Royal Mint at no cost to the taxpayer.
The Treasury said it would not comment on the cost of producing the coins as this is "commercially sensitive information".
Mr Johnson had repeatedly said the UK would leave on 31 October deadline with or without a deal.
However, the law - known as the Benn Act - required him to accept the EU's extension offer when he couldn't get his Brexit deal through Parliament in time for the deadline.