Beware the hot microphone.

That's the lesson for Mike Coupe after the chief executive officer of British grocer J Sainsbury was caught singing "We're in the Money," on camera as he prepared for a television interview.

The gaffe, covered widely in the British media, came after the news that Sainsbury's will acquire Walmart's Asda in a US$10 billion ($14.2b) deal.

Coupe apologised and said it was an "unguarded moment trying to compose myself before a TV interview," according to British press reports.

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Read more: Sainsbury's to buy Asda

"It was an unfortunate choice of song from the musical '42nd Street,' which I saw last year, and I apologise if I have offended anyone," Coupe said in the statement.

Yesterday, Sainsbury's agreed to buy Walmart's UK unit, Asda, for 7.3 billion pounds ($14.2b) in cash and stock in a deal that would create Britain's largest supermarket chain and marks a seismic shift in the country's grocery market.

The deal combines Britain's second and third-largest supermarket chains, giving the combined company 31.4 per cent of the market and putting it ahead of the current leader, Tesco, which has 27.6 per cent, according to data from Kantar Worldpanel.

The company will retain both the Sainsbury's and Asda brands and there are no plans to close any of its more than 2,800 stores, Sainsbury said.