Sainsbury’s failed to warn cat owners that they might be giving their pets toxic food for a month because customers who had opted out of marketing emails were also excluded from alerts about a nationwide product recall.

Batches of Sainsbury’s hypoallergenic cat food, Applaws and Ava are at the centre of a Food Standards Agency investigation, after a spike in cases of a deadly disease called pancytopenia. Thousands of cats are feared to have died from the disease in the past few months.

The probe is focusing on the possibility that products manufactured by Fold Hill Foods for major retailers, including Sainsbury’s and Pets At Home, contained mycotoxins, a dangerous type of mould.

Sainsbury’s and other retailers recalled the product on 16 June but several customers who bought cat food from the supermarket did not receive an email alert until mid-July.

In one email exchange seen by the Guardian, Sainsbury’s told cat owner Linda, 57, a software engineer from Edinburgh, that she had not been sent a product recall because she had asked not to receive promotional emails.

“I can confirm to you that after having the matter investigated, you have not received an email notification regarding our cat food recall as you have unsubscribed from our marketing emails, which is also where our recalls are provided from,” the supermarket said.

Linda, who asked for her surname to be withheld, learned of the product recall from Facebook groups and consumer group Which? and said her cat was fortunately unharmed.

“I saw nothing from Sainsbury’s, social media or otherwise, until an email arrived from them weeks later,” she said.

“I really think Sainsbury’s has let customers down, and their product recall policy needs to be reviewed.”

A Sainsbury’s spokesperson said that “safety is our highest priority” but admitted that a flaw in its product recall protocol meant some pet owners had received no warning for several weeks.

“A systems issue led to a delay in some customers receiving our recent recall notice,” said a spokesperson.

“We put this right as soon as it was identified and we continued to display the recall notice in store and online.”

Members of a Facebook group set up for cat-owning customers of the supermarket have previously complained that signs were often not prominently displayed.

Another customer shared emails that show he raised the alarm with the supermarket on 22 April, saying he believed food bought from the store had made his cat ill.

The supermarket told him on 17 May, a month before Fold Hill Foods recalled several batches of dry food, that “we feel that the cause of the sickness has not come from our product”.

The company told him that its supplier has also reviewed the food and found no issues.

The Food Standards Agency and Royal Veterinary College began investigating the cause of the pancytopenia outbreak in May but have yet to report a definitive cause. They are also looking into whether the cause could be something other than food.

More than 100 cat owners are understood to have contacted law firm Leigh Day, which is exploring the possibility of a class action lawsuit, depending on the outcome of the investigation.