All the big four grocers saw sales fall over the past three months, with Morrisons the biggest loser as its sales fell 2.6 per cent in the 12 weeks to July 14.
Kantar said year-on-year supermarket sales fell by 0.5 per cent over the period – the first overall decline in the sector since June 2016.
The firm said sales are down on last year’s record summer as spending on traditional summer categories like alcohol and ice cream has declined this year.
Fraser McKevitt, head of retail and consumer insight at Kantar, said Morrisons continues to sell more on promotion than any other retailer.
Currently 47 per cent of spend through Morrisons' tills is linked to a deal and this proportion is still increasing in contrast to many of its rivals.
"Morrisons has pursued a very different strategy on promotions compared to everybody else. At a time when the discounters don't do promotions and Sainsbury's, Tesco and Asda have really cut back on promotions over the last couple of years, Morrisons has continued," said Mr McKevitt.
"Nearly half of Morrisons' sales are on promotion and that's always been a key way they've got people through the door, but on their current growth figures, it's not necessarily working.
"People want more than just promotions. They want every day low prices and own label ranges, etc, and I think Morrisons will be looking at those."
He said that the Morrisons shopper is a little bit different to other shoppers.
"They tend to be in slightly smaller households and they tend to be a little bit older on average. Morrisons has obviously taken the decision that the people who come through its doors want those kind of deals," he said.
"Morrisons has had a good run over the last couple of years, but it's currently the least well performing of the big four, which in such a tight race in a market that isn't growing, is always something that any retailer will be looking at very carefully and working out whether to change."
Sales at market leader Tesco fell 2.0 per cent and they were down 2.3 per cent at Sainsbury's. Asda's sales also fell by 2.0 per cent.
Mr McKevitt said Asda’s performance faced a particularly hard comparison against last summer, when sales were growing at their fastest rate in more than six years.
"However, Asda has increased spend among more affluent ‘AB’ shoppers and achieved double-digit online sales growth - two significant boosts," he said.
"Asda is doing a lot of work around ranging in store, particularly around its more premium own label lines which are still growing well. It is getting the basics right.
"It is doing a bit better than Morrisons at the moment, who are suffering a little bit by comparison.
"Asda is working quite hard at click and collect model as well as well as the delivery model. For some people that is a more convenient way to shop so I think that's helping it."
Kantar said that on the back of record sales during last year’s hot summer, the tough period was expected. It anticipates that the market will return to growth once the comparative highs of the 2018 summer pass.
"Is it consumer confidence flowing through? Is it uncertainty around Brexit? On the face of it, there's no evidence of that yet. It's simply the fact that people went out and spent so much in the sunshine last year and there was also the men’s football World Cup," said Mr McKevitt.
"The main driver behind the fall is people are making fewer shopping trips. People aren't popping out. Convenience stores did very well last summer.
"It's annualising against what was a summer party for the supermarkets last summer."
Kantar said households are making one less shopping trip this year, which is enough to tip the market into decline.
Consumers spent £75m less on alcohol this year, with beer sales down 11 per cent and cider down 13 per cent. Soft drinks sales fell by £56m and ice cream by £55m. However, the cooler weather gave confectionery a chance to shine as it racked up an extra £68m of customer spend. Chocolate sales rose 15 per cent as shoppers enjoyed a treat without having to worry about whether or not it would melt.
Lidl was the fastest growing bricks and mortar retailer, with sales up 7.0 per cent. Aldi's sales rose 6.7 per cent.