The problems experienced by shoppers who wanted to pay by direct debit card in Albert Heijn, Etos and Gall & Gall stores on Monday may have cost the company millions of euros in lost revenue, broadcaster NOS said on Tuesday.

The problem was caused by a KPN firewall which filters data traffic and which failed, causing another to take over and block all forms of communication, NOS said.

The problem hit stores all over the country on Tuesday afternoon, causing long waits at cash registers. Some stores closed their doors early because of the payment problems.

On average, the Ahold retail group turns over more than €30m a day and 80% of its shops were open on Whit Monday, a public holiday.

The company said it now plans to meet KPN to evaluate the breakdown, but would not comment when asked if it planned to make a damages claim, NOS said.