Card machines throughout Ireland were down yesterday, causing losses of millions to businesses and sparking fears of a cyberattack.

Shops were forced to put up signs telling customers they could only accept cash. Online payments were also affected. The problem was caused by a system crash at the payment company, Opayo, and also affected the UK.

Spar in Kilmainham, south Dublin, had signs at the tills asking customers to use cash only and said the systems are affected across the country.

It was a scene replicated across the country, as people struggled to find answers as to who to contact.

Opayo, which is based in Newcastle, England, confirmed it is investigating, with many people asking the company if it had been hacked.

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Opayo wrote on Twitter: "Apologies for the disruption to our service, we have encountered an issue which has impacted on transaction processing. Please check our system monitor for updates on this.

"We are currently experiencing an issue where some transactions are not processing successfully. Investigations are underway to resolve the issue."

In reply to queries from several businesses, it said payments that were cancelled when the system went down would remain cancelled and retailers could check individual transactions on their Opayo payment portal.

A spokesman for Opayo's parent company, Elavon, said: "We apologise for any inconvenience this may cause and will share updates as more information is available."

The Banking and Payments Federation Ireland confirmed it is investigating.

One woman, Tina-Marie O'Neill, tweeted: "Cash only at local filling stations in South Co. Dublin. e-Point of sale machines out of service this evening. Another cyberattack?"

The bookshop at Maynooth University also tweeted to AIB saying it could not process any payments.