Grocery sales in April were hit by unpredictable weather and rose at the lowest rate in almost 15 months, and Asda outperformed suitor Sainsbury’s.

Sales rose 1.2% year on year in the four weeks to April 21, the smallest increase since January 2017, Nielsen data showed.

The number of items purchased was down 1.1% year on year, the worst performance for more than seven months.

The study was released a day after Asda and Sainsbury’s revealed merger plans.

Nielsen’s data showed Asda achieved the best year-on-year sales performance among the big four with a 2.3% advance. That gave it a market share of 13.9% and would mean a combined market share with Sainsbury’s of 28.8% if the pair’s planned merger went ahead.

That would put the enlarged business ahead of Tesco, whose market share stood at 27.1%.

Sainsbury’s sales grew 0.2% year on year, while Tesco’s rose 2.1%.

Nielsen’s UK head of retailer insight Mike Watkins said: “Both retailers [Asda and Sainsbury’s] have over half of all households as shoppers every 12 weeks, while over three-quarters also visit Tesco.

“This illustrates how competitive the retail landscape is, which is why we’re seeing further consolidation within the industry.”

Grocery market share figures from Kantar Worldpanel also published today showed market shares of 15.9% and 15.5% respectively for or Sainsbury’s and Asda, giving a merged business a potential share of 31.4%.

Over the past 12 weeks, Sainsbury’s sales inched up 0.2% while Asda’s rose 1.4% according to Kantar.

Market share was down year on year at each – 0.3 percentage points at Sainsbury’s and 0.1 at Asda.

Kantar Worldpanel head of retail and consumer insight Fraser McKevitt, said: “A merger between Sainsbury’s and Asda would transform the traditional landscape, placing nearly a third of market share in the hands of the joint supermarket giant, though the march of the discounters – and any enforced store closures – could impact this figure.

“The two supermarkets appeal to different customer bases. Asda achieves nearly two-thirds of its sales outside London and the Southeast of England, in contrast to Sainsbury’s, which registers 59% of its sales in those two areas.”

He added: “15.8 million households bought their groceries at Asda over the past 12 weeks – 500,000 more households than shopped at Sainsbury’s – but we are seeing a substantial number of customers frequenting both retailers.

“Nearly 9 million households visited both Sainsbury’s and Asda, with consumers showing little retailer loyalty.”

Outside the big four, Aldi generated year-on-year growth of 10.3% and Lidl was up 9.3% according to Nielsen.

Iceland delivered the best growth outside of the discounters, with sales up 3.6%.

Watkins said that grocery retail sales momentum should be “kick-started by any sustained period of warm weather and the proximity of the two bank holidays in May, plus the royal wedding.”

He said: “This is all good timing, with the activation of promotions around the football World Cup kicking off soon after. These three events should be enough to return the industry to volume growth during the next eight weeks.”