Thousands of online and mail order retailers have been unable to receive payments after an upgrade at online payments service Protx went wrong.
The service is used by online retailers across the UK
Protx said its service was back online by 1430 BST.
"We apologise wholeheartedly for the performance... throughout the morning and afternoon," it said on its website.
But retailers told the BBC they had been left unable to do business from 0600 BST that morning, and that problems persisted throughout the day.
Jamie Chapman, whose family firm upliftedlingerie.co.uk uses Protx's services, said the company had failed to keep its customers properly informed.
"I've been sitting here watching orders come in, and they just fail and fail," Mr Chapman said.
"We're a small family firm, and this is really hurting us."
His firm had lost about 50 orders worth an average of £50 each, he said, regardless of potential loss of future business.
Protx handles secure credit and debit card payments for more than 10,000 retailers, and is the largest independent payment service in the UK.