American Express customers trying to shift debt to a new 0% balance transfer credit card might be rejected as a result of a technical glitch.

American Express cards are hugely popular for the rewards, but they seldom come with any 0% introductory period on purchases or balance transfers and tend to charge hefty APRs.

So, many American Express cardholders who have racked up substantial debt will be looking to move it to another provider offering a 0% balance transfer deal.

But a little-known incompatibility between American Express and certain card providers’ platforms means the transfer could fail and leave you lumbered with a 0% balance transfer card you can't use.

What’s the problem?

The majority of UK card providers issue cards with 16-digit numbers, while American Express cards are issued with 15 digits only, on the front of the card.

Because American Express cards are still less common than MasterCard or Visa cards, some computer systems and payment platforms are designed to expect 16 digits as standard. 

As a result, when 15 digits are entered into the system, it can reject the balance transfer on the basis that it simply does not accept the card.

Read more: John Lewis offers £20 reward when you apply for its credit card

What Sainsbury’s Bank says

Sainsbury’s Bank admitted to personal finance website loveMONEY that there is occasionally a problem with American Express cards, and referred them to their ‘Exclusions’ page online, which does indeed say that Sainsbury’s Bank “may not be able to process balance transfers from some providers”, but does not explain why.

A spokesperson for Sainsbury’s Bank told loveMONEY that there could indeed be an issue but refused to elaborate on what the technical difficulty is, because “Sainsbury’s Bank are working to fix it”.

Confusingly, American Express said that there shouldn’t be an issue.

What to do if you have American Express card debt to transfer

If you’ve got debt on an American Express credit and want to shift the balance be cautious about where you apply and make sure they can facilitate a transfer. 

Some providers like Tesco Bank, Post Office and Nationwide have platforms that are set up for American Express balance transfers according to the research by Balance Transfer Expert.

To be sure, try ringing up the provider and double checking they can perform balance transfers from American Express cards.