Supermarkets have reacted angrily to research by Which? suggesting online groceries have a shorter shelf life than store-bought goods.
Online grocery shopping has become increasingly popular
A snapshot survey by the consumer organisation found "best before" dates were, on average, a day earlier for goods ordered over the internet.
But supermarkets have criticised the group for using a small sample.
Researchers bought the same 10 grocery items in-store and online from four different supermarkets.
Which? bought goods from Tesco, Sainsbury's, Asda and Waitrose but said none of the chains surveyed performed worse than the others overall.
It found that the average use-by dates on goods such as carrots, potatoes and salads bought in store were more than a day later than for online groceries.
One loaf of bread bought in store had a shelf life that was eight days longer than that of the bread provided by the store's internet service.
"Online grocery shopping might save you from traipsing around a busy supermarket, but you may have to eat your food sooner than if you shop in the store," said a spokesman for the consumer group.
The research supports a Which? report from March 2007 which found about 60% of Asda and Sainsbury's shoppers believed food bought online was not as fresh as that selected in a store.
Four in 10 customers of Waitrose Deliver and its partner Ocado, which also delivers Waitrose goods, agreed.
But supermarkets have responded angrily to the survey.
"We think the research by Which? into use by dates is seriously flawed," said Asda spokesman Dominic Burch.
"They only shopped once in each store and once online, and even then the difference they found was marginal.
"It's daft to think that every online retailer in the land is deliberately trying to give their online customers a worse deal than those who shop in store," he added.
Asda, Sainsbury's, Tesco and Waitrose insist staff are trained to chose the freshest items available for internet customers.
"Sainsbury's uses personal in-store shoppers who take products from the shelves throughout the day in exactly the same way a customer would," said a Sainsbury's spokesman.
Tesco said its online customers can write notes on their order requesting minimum best-by dates.
Waitrose also questioned the accuracy of Which?'s research.
"It was based on one shop, of 10 items. We would be surprised if this snapshot was representative," said a spokesman.
Jason Gissing, one of the founders of Ocado, said Which?'s criticism ignored the difference between his business model and those of the other supermarkets.
"We don't have in-store customers who take the latest freshest product leaving older stock for dotcom customers," he said.
"We aggregate many stores into one location ensuring a very high turnover of stock.
"We never deliver stock that is in its final day and in over 95% of the time we make sure it's never less than 50% of the absolute best you could get", he added.
But Which? defended its findings.
"We never claimed it was a major piece of research," said a spokesman.
"It is a snapshot survey which appears to back up anecdotal evidence we've heard from consumers."
Which? may do a more extensive survey in the future, he added.