Online retailers have been reporting worrying shortfalls in their orders this week after hackers wreaked havoc with credit card systems. In one of the busiest weeks pre-Christmas, attacks on MasterCard, PayPal and Amazon, the freezing weather and unrelated issues at the processing intermediary Sage Pay have left many online merchants far short of expected sales.

Emma-Louise Ewing, owner of the Kitty Cat Boutique in Falkirk, said she first noticed a problem on Wednesday, the day hackers sabotaged MasterCard's website in revenge for its decision not to take donations to WikiLeaks. "My credit card sales just dropped off to zero," she said. "I know I'm a small business, but I do expect to get orders every day."

At first, she thought the fall might have been related to the bad weather, which has been affecting deliveries all week in Scotland, but it was only when she started to receive payments by cheque in the post that she realised the extent of the problem. "I'm still getting the same number of web hits I would expect at this time of year, but unfortunately nothing like the same volume of business," she said.

While her sales have slowly begun to recover, she said they are not yet back to normal and that her website does not enable her to contact customers who have tried but failed to process orders. "Unless customers contacted me directly, I'd have no way of knowing whether they're having problems," she said.

Sellers trading through Amazon, another target for the hackers, have also reported serious disruption. Richard Stubbings, who runs Kulture Shock, an action-figure and film-memorabilia retailer trading mainly on Amazon, said he had also noticed sales drop off dramatically from Wednesday onwards.

"I get the stats from Amazon every day, and this week they went down from 157 sales a day to 44," he said. "There's no way I'd expect that level of decrease at this time of year."

Stubbings was also one of many retailers who experienced problems earlier in the week because of a loss of service by the credit card payments processor Sage Pay.

Simon Black, managing director of Sage Pay, which serves 32,000 online retailers in the UK, said its systems had gone down for four hours on Monday evening, but that was caused by a separate hardware flaw unrelated to the MasterCard attacks. "Our customers also experienced problems processing MasterCard transactions throughout most of Wednesday, and that was a direct result of the attacks," he said.

Black said Sage Pay was processing sales as normal again on Thursday, and that by yesterday it was handling "record volumes" of transactions.

Gareth Mitchell said his ethical gifts business, Tree2mydoor, in Manchester had been badly hit by payment processing failures all week. "People have been unable to complete transactions since Monday," he said. "It should have been our biggest day of the year but sales were down massively when they should have been up. We just couldn't understand what was wrong."

Mitchell said he had been able to phone some customers back and ask them for alternative payment methods. "Perhaps because we're an ethical business, we've found our customers to be quite understanding about the situation, but you still have to be careful what you say," he admitted. "One of the biggest problems is the minute you mention the word 'hacker' or anything like that, it freaks people out. They often just decide not to use their credit cards at all and buy their presents at the shops instead."

A spokesman for Paypal UK said: "The action by protesters has had some effect, but the site has been up and running throughout. The service has been slower, but that is because it's a very busy time of year."