Marks and Spencer Group PLC (LSE:MKS) shares climbed on Wednesday morning but quickly dropped back as it put out a statement to reveal some products are running short in its stores after the ongoing cyber-attack.
The FTSE 100 clothing and groceries retailer said the cyber issues that first emerged over a week ago had led to "pockets of limited availability" of products.
A decision has been made to "take some of our systems temporarily offline" to try and manage the incident, it added.
"We are working hard to get availability back to normal across the estate."
Earlier, shares bounced back up for the second day in a row, despite having suspended online orders since last Friday.
M&S's cyberattack was reported to be linked to the hacking group Scattered Spider, involving ransomware that may demand up to £10 million to unlock the retailer’s systems.
This has suggested that "investors are confident it can sort out disruption to systems from a cyber-attack and that too much bad news has been priced into the stock", said Russ Mould at AJ Bell.
"The company hasn’t taken any online orders for the past five days which could add up to a significant number of lost sales.
"Its reputation may also have been damaged as shoppers worry if their details have been stolen by hackers and whether it is still safe to use cards in-store.
"The fact the disruption is still unresolved would suggest that management are being incredibly thorough behind the scenes to ensure everything is secure before switching online platforms back on."
M&S is reported to have enlisted cybersecurity experts from CrowdStrike, Microsoft and Fenix24 to help investigate, but has not provided a timeline for resolving the issue.