Money expert Martin Lewis has warned anyone with a Cash ISA not to break the rules by accident and end up losing your entire £20,000 deposit limit.

There’s just hours to go until Cash ISA limits reset just after midnight on Sunday morning, and if you miss the deadline, you lose out on up to £20,000 of tax-free savings you could have made before the limits reset. If you time it right, you could deposit £40,000 tax-free savings between now and Sunday – £20,000 before Sunday, and then another £20,000 on Sunday, after the yearly limit resets – for a total of £40,000, all protected from tax.

With so many looking to max their ISA savings now, lots of banks and building societies are offering great rates to try to tempt savers to switch with rates as high as 5%.

It’s possible to swap an old ISA, paying a poor interest rate, into a new one instantly before the deadline – but if you do it incorrectly, you will lose all of your £20,000 limit, Martin Lewis warned.

Writing on his MSE website, Martin Lewis said: “Many old ISAs now pay appalling rates, so check yours now. If they don’t come close to the current top picks, you can transfer to boost them, but you must do it correctly.

“Consolidating new and old cash ISAs into one new shiny ISA makes it much easier to transfer again in future. You can also transfer an ISA without paying any new money in.

“Transferring an old ISA is a technical process – it’s not just like switching a normal savings account. Yet as long as you abide by the golden ISA transfers rule, it should go smoothly.”

But Martin warned that if you withdraw the money instead, you lose all your tax benefits immediately.

He warned: “If you want to transfer, never withdraw money from a Cash ISA! You’ll immediately lose all the lasting tax benefits!

“Instead, speak to the new provider and fill in an ISA transfer form. Your new provider should then sort it all out, including moving the money over for you – keeping your ISA cash permanently tax-free.”

Before you transfer, though, check you won’t be penalised by your current ISA provider.

Martin added: “You may be charged a penalty by your current provider for transferring out. This is not common these days (except on fixed ISAs if you leave during the term), but always check, especially if your accounts are old.”