A popular cryptocurrency exchange has warned customers that their Bitcoin could be stolen after hackers managed to create a backdoor in the service.

Bitpay said that customers using earlier versions of the app should “assume” that an attacker had stolen the private keys needed to access their cryptocurrency.

The attacker - or attackers - are understood to have infected a code library used by the service several weeks ago and have been quietly siphoning the keys unbeknown to the owners.

Customers using Android and iOS versions 5.0.2 through to 5.1.0 of its Copay app are affected, it warned. They were advised to update the app and move Bitcoin to a new wallet.

A spokesman for the US-based payments company said: “Our team is continuing to investigate this issue and the extent of the vulnerability. In the meantime, if you are using any Copay version from 5.0.2 to 5.1.0, you should not run or open the app. A security update version (5.2.0) has been released and will be available for all Copay and BitPay wallet users in the app stores momentarily.”