The departure of Asda's chief operating officer David Cheesewright on 4 January will come as a huge blow to the retailer, say analysts.
Cheesewright is taking over as president and CEO of Wal-Mart Canada from Mario Pilozzi, who is retiring after seven years at the helm. He worked with Pilozzi in Canada for a year as COO, before returning to the UK in 2005.
Since then, he has played a key role in getting the business back on track and analysts have questioned whether it will be able to keep up the momentum without him.
"Cheesewright wore the trousers at Asda and his departure will have created a big hole in the Asda board," said Shore Capital analyst Clive Black.
Another analyst added: "Losing Cheesewright is pretty serious when Asda still needs to get back to firing on all cylinders and finding a replacement will be tricky with a lack of good trading operators around."
Cheesewright said he was unable to turn down the presidency, despite the fact the business was less than half the size of Asda.
"Wal-Mart has made no secret that it wants to grow food sales in Canada and that's where my background lies. Only 18% of the Canadian business is food so there is a lot of growth to come," he said. "This is a huge opportunity. From a C$1bn operation when Wal-Mart took over in 1994, it is now a C$16bn (£7.75bn) operation and one of the most profitable Wal-Mart businesses."
However he admitted to mixed feelings over his departure. "I enjoyed my time at Asda. We had a great team," he said. "The day I landed two years ago, we made a lot of redundancies, we were losing market share and missing plans. Now we are Britain's fastest-growing retailer and hitting all our sales plans."
Cheesewright originally joined Asda from Mars Confectionery in 1999.