Sales at all the big four supermarket chains fell over the past three months, hit by poor weather and tough comparisons with last year.
The latest figures from Kantar show that Morrisons was the worst hit, with sales down 2.7 per cent in the 12 weeks to August 11 as shoppers put fewer items in their baskets.
Fraser McKevitt, head of retail and consumer insight at Kantar, said: "Morrisons' fundamental problem is average basket sizes are getting smaller. People are picking up fewer items.
"It is upping the level of promotions, so 46 per cent of what Morrisons sells is on some kind of promotion, which is way higher than the average for the big four, which is under 40 per cent.
"Morrisons is increasing the level of promotion, but it's not producing a sales response. If you compare that with Sainsbury's, which is upping the level of promotion, Sainsbury's is doing the best of the big four."
Sainsbury's sales fell just 0.6 per cent over the three month period, making it the best performer of the big four.
Asked why Morrisons' basket sizes are smaller, Mr McKevitt said: "It must be to do with the fact that it's not fulfilling what shoppers want in terms of the products that are on offer.
"It's a combination of that with the price. It's something that Morrisons needs to address pretty quickly. It also needs to find a way of attracting new shoppers in to its stores.
"It's been a difficult couple of months for the Morrisons business."
Analysts pointed out that Morrisons' sales this summer were up against very tough comparatives. Over the three month period last year, Morrisons was the best performer of the big four.
A spokesman for Morrisons said: "It's important to note that these figures compare us to a period last year where we saw particularly strong sales, our best for nine years.
"We remain focused on becoming more competitive for our customers and that has involved a mixture of price reductions as well as promotions. We're puzzled about Kantar's commentary on our level of promotions because we don't recognise their figures."
Asda and Tesco’s sales shrunk by 1.5 per cent and 1.6 per cent respectively although Kantar said Asda’s online growth of 11 per cent was notably strong.
Mr McKevitt said: "Asda is in a good place. Its typical customer is a family who are looking to get big, weekly shops."
Asda's online sales are also seeing a boost from the popularity among shoppers of click and collect.
"Asda is following the concept where people pick up deliveries rather than get them delivered to the house," said Mr McKevitt.
"It works for some shoppers as it's more convenient than waiting in. It certainly works for supermarkets as it costs a lot less for people to come and pick up their shopping rather than get a driver to take a great big truck round to their house."
Asked why Asda's sales had fallen 1.5 per cent, Mr McKevitt said: "Asda is pretty much trading with the market. It's really tough for everybody. Last summer was very hot and we also had the football related promotions which particularly encouraged sales of barbecue items, soft drinks, crisps and snacks.
"The market is in the doldrums a little."
Overall, UK supermarket sales were flat over the three month period.
Mr McKevitt said: "The memory of last year still looms large for retailers and this summer's comparatively poor weather, combined with low levels of like-for-like price rises, have made growth hard to find for retailers.
"July's hottest day on record wasn't enough to shift the market into growth, but the grocers will have been encouraged by glimpses of better weather during the past four weeks which helped boost sales of summer staples like hayfever remedies, suncare and burgers by 17 per cent, 8 per cent and 5 per cent respectively."
Kantar said Ocado was the fastest-growing UK grocer, reporting a 12.6 per cent rise in sales although the online supermarket only accounts for 1.4 per cent of the market.
Lidl's growth was the next fastest, at 7.7 per cent, to give the discounter a record total market share of 5.9 per cent.
Rival Aldi posted a 6.2 per cent increase in sales, supported by an 11 per cent rise in bakery products and a 13 per cent increase in biscuits.
Nearly half of all households in the UK shopped in an Aldi during the last 12 weeks.