Tesco is the UK's most successful supermarket group

That is the conclusion of some mystery shopping carried out each week for The Grocer trade magazine.

Over the last six months its shoppers found that at Tesco they could only fill their basket of 33 basic items on just one out of every four trips.

Tesco said the survey was "small and unrepresentative" of the true picture.

Morrisons by contrast was the best performing supermarket across the UK over the last six months, The Grocer said.

In The Grocer's latest weekly survey, now in its fifth year, up to nine of the items on its basic shopping list of 33 were out of stock or unavailable at Tesco.

Shortages

Chris Fletcher, author of The Grocer report, said the problem appeared to be a mystery, especially as all the supermarkets have invested millions in technology to avoid product shortages.

PRODUCT AVAILABILITY SCORES Morrisons - 75% Waitrose - 61% Asda - 57% Sainsbury - 40.5% Co-op - 33% Tesco - 25% Safeway - 24% Somerfield - 21% Source: The Grocer

"We cannot really put our fingers on it," said Mr Fletcher.

"We talk about the last 50 yards of getting products from the warehouse onto the shelves as being the most difficult."

In a statement, Tesco said its own customer surveys painted a different picture.

"This small and unrepresentative survey by The Grocer covers very few products in a tiny proportion of our 850 stores," it said.

Getting better?

"We do our own comprehensive research across all of our products in all of our stores to keep track of availability and our customers tell us it is good and getting better."

Tesco added: "That is why our recent trading statement shows that more and more customers are coming to shop in Tesco."

Over the last six months The Grocer's product availability score for Tesco is just 25%, although ahead of some competitors.

Morrisons was on top with 75%, with Asda on 57% and Waitrose on 61%.