Britain's iconic £1 coin is to be phased out after more than three decades in circulation by a larger 12-sided replacement, which launches tomorrow (Tuesday 28 March). Here's what you need to know.

The new £1 - introduced to clamp down on counterfeit coins - has been billed as the most secure coin in the world and contains security features such a hologram that make it more difficult to produce fakes.

Both the old and new coin will co-exist together for a period of around six months, until the round pound ceases to be legal tender on 15 October 2017.

If you're an avid piggybanker, it's best to spend or deposit your old-style coins ASAP - read our Coin Jar Warning for more on this.

Not all machines are ready for the new coin

While many of the country's millions of coin machines have been upgraded or replaced to accept the new £1, not all have yet - meaning you may struggle with tasks such as buying snacks or paying for parking in the coming months.

Here's what to expect:

  • Supermarkets: The UK's biggest supermarket chain Tesco has confirmed it will unlock trolleys at up to 200 stores until their locks can be overhauled to take the new coin. All Sainsbury's and Asda trolleys are already set up to accept it. In all cases the old-style coins can still be used in the new locks.
  • Vending machines: When the new coin goes into circulation, around 85% of vending machines will be able to accept it, according to the Automatic Vending Association (AVA) - and all will still accept the old round-sided coin. The industry body estimates that all of the UK's 500,000 machines will be full upgraded by 15 October.
  • Car parks: A spokesperson for the British Parking Association (BPA) said the majority of parking machines are ready for the new coin, but some older machines can't be upgraded and so new equipment may be required. All machines will continue to accept the old coin.
  • Trolley tokens: Businesses which make pound-shaped tokens for supermarket trolleys, such as Lancashire-based Pound for Life, have now begun manufacturing them in the new 12-sided format. However the old-style round tokens will continue to work in supermarket trolleys after they've been upgraded.
  • Banks: If you have old-style £1 coins and you want to deposit them in your bank or building society account, you're advised to do so before they cease to be legal tender on 15 October.

    However some major banks and building societies - including the Bank of Scotland, Barclays, Clydesdale, HSBC, Lloyds, Natwest, the Post Office, RBS, Santander and Yorkshire Bank - have confirmed they will continue to accept the coins from their own customers after this date.
  • Amusement arcades and casinos: Amusement machine industry body BACTU told us many arcade games and coin machines have already been updated to accept the new £1 alongside the old one, and it's confident they'll all be done by 15 October.

    In many cases change machines are being modified to ensure they only dispense the new-style coin, with the round pound hived off into a separate chamber and taken straight to the bank.

New 12-sided £1 coin enters circulation – what you need to know
The round pound is being replaced by the larger 12-sided £1 coin, which is based on the old threepenny bit

Spend or save your old coins before 15 October

If you’re left holding old pound coins after they stop being legal tender on 15 October and your bank will no longer accept it, opening a new account with a different provider may be the only solution.

In any event, carting a bag of coins to the bank is likely to be a real faff – particularly if there isn’t a branch near where you live. So it’s much better simply to spend or save them now.

If you’re saving for something specific though, don’t feel this warning means you need to abandon your piggybank savings plan. Simply take the pound coins out and replace them with a note or other coins of a similar value – then keep on saving.

New coin is thinner but larger

The new coin has a gold-coloured outer ring and a silver-coloured inner ring - rather like the current £2 coin - and is based on the design of the old 12-sided threepenny bit, which went out of circulation in 1971.

Its measurements differ too - it's thinner and lighter than the round £1 coin, but slightly wider.

It could be weeks before you see one

While banks and retailers will start to receive the new £1 coin from tomorrow (28 March) it will take about a month for the new coins to reach most bank branches and retailers.

Additional reporting by the Press Association