The boss of Marks & Spencer has told customers the retailer is “working day and night” to manage the cyber attack that has forced it to temporarily shut down its online operations.

Stuart Machin said he was “really sorry” for the disruption to services, but did not say when normal business would resume.

M&S’s problems began over the Easter weekend, and last week it confirmed it was dealing with a “cyber incident” and paused online orders on its website and apps.

The Co-op and luxury department store Harrods have also been targeted and the National Cyber Security Centre (NCSC) has urged retailers to be vigilant.

In his message, Mr Machin said: “We are working day and night to manage the current cyber incident and get things back to normal for you as quickly as possible.”

He also thanked customers for “all the support you have shown us”.

The cyber attack has already had a significant impact on the retailer, as online transactions account for about of third of its clothing and home sales. On average, £3.8m is spent on clothing and home products on its website and apps every day.

There have also been gaps on the food shelves in some stores after the firm took some of its systems offline in an attempt to manage the attack, while its loyalty scheme and gift card payments have been disrupted too.

Analysts have pointed out that M&S risks losing out to rivals, as online shoppers can quickly switch to other retailers’ websites.

The problems have also coincided with a period of warmer weather, when people are likely to want to buy new summer clothes.

M&S shares dropped almost 2% on Friday morning, and are now down about 5% from the point when the company first disclosed its IT systems had been disrupted.

With both the Co-op and Harrods also facing cyber attacks this week, the NCSC – the UK government body responsible for supporting organisations facing cyber threats – has extended its warnings about such attacks to all organisations in the UK and urged them to be vigilant.

NCSC chief executive Richard Horne said the recent spate of attacks should serve as a “wake-up call” for big retailers.

He said the NCSC was working closely with the companies that had reported incidents, “to fully understand the nature of these attacks and to provide expert advice to the wider sector”.

Helen Dickinson, chief executive of the British Retail Consortium, said she was worried about the attacks and acknowledged it was concerning for customers and businesses.

She told the BBC’s Today programme that big retailers could be vulnerable to such attacks because they were “high-profile businesses, we all interact with retail each and every day”.

In addition, she added that they were “highly digital businesses, not just from an online shopping point of view but retailers have lots of systems – payments systems, stock systems”.

However, she said that all of the businesses affected so far have been able to keep their stores open.