Some holidaymakers who've taken the new Supercard with them to spend overseas have found their personal identification numbers (PINs) don't work and their cards have been cancelled without warning, with replacements sent to their home address.

Issued by currency bureau Travelex, the Supercard is a hybrid card that anyone can get, as there's no credit check. It lets you link your debit and credit cards to it using an app, has no added fees on spending and gives one of the best exchange rates on overseas spending.

Travelex said it's cancelled a "small proportion" of the cards – though it refused to say how many – earlier this week after a problem caused users' PINs to stop working.

The firm says it's now notified all those affected by email or text, and is "doing everything we can to assist" users currently overseas – but it wouldn't elaborate on exactly what that help would be.

Use our TravelMoneyMax tool to compare the best rates on your holiday cash.

What is Supercard?

First piloted last year, Supercard's an alternative to cheap overseas credit cards. To get it you need its Android or Apple app on your mobile phone – once you've got that and set it up, it'll send you a card. It then works as follows:

a) You link your existing debit or credit cards to it (not Amex).
b) You spend on it.
c) Then it charges your existing card in pounds, having done the conversion at the near-perfect Mastercard wholesale rate, which is far better than you would've got spending directly on most cards.

However, cash withdrawals overseas have a 2.99% fee (more than the cheapest specialist credit cards) so it's best to spend on the card, not withdraw cash and spend that.

You can see all the Supercard need-to-knows in our Travel Credit Cards guide.

What happened with the cancelled cards?

One Supercard user tweeted founder Martin Lewis on Wednesday to say her Supercard had stopped working while she was on holiday:

Others reported their cards had stopped working with no warning:

Travelex told us it has now notified all those affected via email or text. The email stated:

"It's just been brought to our attention that the PIN assigned to your card now won't work when you try to use your Supercard. But don't worry, we're on the case.

"We're reissuing you with a brand new card, which we're sending to your registered address via first class post, so [it] will be with you within the next few days. We've deactivated your current Supercard so that when your new Supercard arrives, all you need to do is activate it via the app. Your debit/credit cards will still be linked so you won't need to worry about re-linking those.

"Should you be travelling imminently or already be abroad, give us a call on +44 (0) 20 3364 5155."

I've been affected. What should I do?

As above, if you've received an email or text telling you your card's been cancelled, your replacement card should arrive by post at your home address within a few days. You then need to activate it via the Supercard app on your phone.

If you're abroad or about to travel, you can contact Travelex on +44 (0) 20 3364 5155 for more immediate help.

What does Travelex say?

Travelex wouldn't specify in detail what assistance it would offer but a spokesperson told us its customer service team "will provide advice on how they can help so that you can still benefit from the Supercard savings".

The firm added: "For those who are abroad we're doing everything we can to assist them. All they need to do is contact our customer service team."