Aldi, named recently by Which? as the UK’s cheapest supermarket, has now been declared the worst for online order substitutions by the consumer group.

A report covering grocery deliveries, which have shot up in demand over the past two years because of the COVID-19 pandemic, also found that substitute foods across the sector did not always meet dietary requirements or the food intolerance needs of the customer.

Which? said that its survey of online shoppers found that Aldi, which has a click-and-collect online shopping service only, was the most likely of the nine online supermarkets, by a narrow margin, to put substitutions in customer orders.

It found that 49% of Aldi customers had received a replacement item in their most recent shop.

The chain was followed close behind by Sainsbury’s at 48%, Asda at 45%, Morrisons at 43% and Ocado at 41%.

Tesco, the UK’s largest grocer by market share, had substituted at least one item in the most recent order for 39% of the customers surveyed.

The report said that a contender for the most unusual substitution for an Aldi customer was Ben and Jerry’s Phish Food ice cream instead of the breaded fish fillets they had ordered.

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Another Aldi shopper told Which? they had been confused when they had received cooking oil instead of milk.

One shopper reported ordering Sainsbury’s sponge scourers but instead received a Victoria sponge cake.

Another said they had received beef stock instead of brandy butter.

In the case of Morrisons, one customer said their order for a Domestos cleaning product was swapped out for a bottle of orange squash.

One Waitrose shopper received tampons instead of shaving cream, while a Tesco customer was treated to duck paste instead of duct tape.

But not all replacements were disastrous – with one lucky Asda customer receiving three £60 bottles of whisky to replace three £25 ones that were out of stock.

Supermarket pickers – the people who choose online shoppers’ items from supermarket shelves or warehouses – often receive automatic prompts on their handheld scanners about what to choose as a substitution if the item wanted is not available, Which? said.

Its report added that some of the computer-generated ideas were clearly not suitable and had to be manually overridden, but sometimes the pickers have to give up on choosing a sensible swap because of time targets.

Which? said some swaps were clearly inappropriate – with meat being substituted for vegan products – and on food intolerance grounds.

One shopper who had their gluten-free plain flour substituted with regular self-raising flour told Which?: “It is an allergen food, not just any food.”

Which? retail editor, Ele Clark, said: “While product substitutions in your online shopping can sometimes be genuinely helpful, our research has shown that they can also be downright ridiculous.

“You do have the right to reject substitutions at the point of delivery, or you could opt out of receiving substitutions altogether – though this can result in a real headache if the key ingredient for your dinner that night is missing.

“If you do end up with a substitution that you don’t want, always contact the supermarket and ask for a refund.”