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Asda has overtaken Sainsbury's to become the UK's second-largest supermarket, figures suggest.

Asda's sales rose 0.1% in the 12 weeks to 24 March taking its market share to 15.4%, research firm Kantar said.

In contrast, Sainsbury's sales fell 1.8% over the same period, meaning its market share dropped to 15.3%.

The two supermarket groups are currently struggling to persuade the UK competition watchdog to allow their proposed £7bn merger to go ahead.

The duo have argued that the tie-up will save them £1.6bn and allow them to pass on £1bn 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.

Kantar said one reason for Sainsbury's sales slide was that much of its non-food was now sold via catalogue-retailer Argos, which the supermarket group bought in 2016. Argos sales are not included in Kantar's figures.

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

"Thirteen million households visited Aldi at least once in the past 12 weeks - now more than those shopping at Morrisons," said Fraser McKevitt, consumer head of retail and consumer insight at Kantar.

Overall, year-on-year supermarket sales over the 12 week period rose 1.4% - the slowest rate of growth since March last year, which was partly due to the late Easter meaning that Mother's Day fell outside the reported period, Kantar said.

Despite Easter being later than usual, Kantar said its data showed that shoppers have already spent £146m on Easter eggs this year, while 42% of households had bought hot cross buns.