Asda has reportedly overtaken Sainsbury's to become the U.K.'s second-largest supermarket, with the former's shares rising by 0.1% in the 12 weeks to March 24, according to the BBC, which cited figures from research firm Kantar.

It said the figures indicate its market share is now 15.4%. In contrast, Sainsbury's sales fell 1.8% over the same period, meaning its market share dropped to 15.3%.

Tesco is the U.K.'s largest supermarket group, with almost a 28% market share.

Asda and Sainsbury's are currently struggling to persuade the U.K. competition watchdog to allow their proposed £7 billion merger to go ahead.

The duo have argued that the tie-up will save them £1.6 billion and allow them to pass on £1 billion in price cuts to savers. They have also agreed to sell between 125 and 150 supermarkets and a number of convenience stores if allowed to merge.

According to Kantar's figures, Sainsbury's was the worst performer of all the big four supermarkets, which includes Tesco and Morrisons. Sainsbury's sales fall meant its performance lagged behind smaller rivals, such as Iceland and Co-op.

Discounters Aldi and Lidl continued to expand their reach, with both expanding their market share to 8% and 5.6% respectively in the period.

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