Martin Lewis' Money Saving Expert team has issued an urgent warning to anyone who banks with M&S.
Martin's team included the warning in the latest weekly newsletter and told people they have to act now.
M&S are closing all their current accounts on Tuesday August 31, which means customers have only days left to switch.
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The M&S Current Account, M&S Premium Current Account, M&S Premium Current Account with insurance and Monthly Saver accounts linked to current accounts will all be closed on August 31.
The bank said the move is due to customers switching to online banking, during the last year in particular.
Any requests to switch away from M&S Bank after 31 August will be denied.
Martin's team said: "If you're an M&S Bank customer, you can continue to use your account as normal and view it on internet banking or via the app up until account closure on 31 August.
"But if you want to switch - and this is likely to be the easiest option for those who regularly use their account - you need to start the process by or on 31 August, as it takes seven working days, with a switch date of no later than 10 September.
"You can use the Current Account Switch Service (CASS) to switch to an existing current account you have or to a new current account. This will ensure all your direct debits, standing orders and incoming payments are moved over to your new account.
"If you have money in your account and don't switch, your funds will be automatically sent to you via cheque after account closure on 31 August. It's unclear how quickly this cheque will arrive, so it may be best to withdraw funds now if you need immediate access."
But not all M&S customers are affected, those with credit cards, loans, insurance products and savings other than the monthly saver linked to current accounts aren't affected.
The travel money service will also continue with M&S Bank's 100 travel bureaux remaining.
