In a surprise move, Asda has announced that chief executive Sean Clarke is to step down from his role at the end of the year after less than 18 months in the job.
He will be replaced by Roger Burnley, a UK supermarket veteran who is widely respected in the market.
When Mr Clarke rejoined Asda in July 2016, it was always intended that Mr Burnley would take over the reins eventually, but Mr Clarke was expected to stay in the role for longer.
Retail analyst Clive Black at Shore Capital said: "The timing is certainly surprising.
"Sean Clarke came into a very difficult situation that Andy Clarke (his predecessor) also faced. Walmart (Asda's US parent) had starved Asda's resources and also its management team and Asda deteriorated in the face of competition.
"Sean Clarke deserves credit for what he's done with Asda. He was given more resources than Andy Clarke. That all said, I still think it's surprising that Sean Clarke is going now.
"Roger Burnley is a big blessing for Asda - he's a very capable guy. I'm not surprised he has been put in charge."
Mr Burnley, who is currently Asda's chief operating officer, will take up the role from January 1.
Under Mr Clarke, Asda has seen a return to sales growth after a number of disappointing years. The firm is battling against stiff competition from discounters Aldi and Lidl, which are around 15 per cent cheaper than Asda, which is the cheapest of the big four grocers.
Asda reported its first positive sales growth in three years in August, driven by food price inflation and lower prices. It said like-for-like sales rose 1.8 per cent in the three months to June 30.
This was the first positive quarter after 11 consecutive quarters of diminishing sales as shoppers fled to Aldi and Lidl. It was also a vast improvement on the 7.5 per cent fall that Asda reported over the same period last year.
Asda has been the worst performer of the big four for some time although there have been signs of improvement recently.
Mr Clarke has been credited with reducing prices, getting rid of superfluous stock and introducing discount level brands such as Farm Stores.
In an internal memo, Dave Cheesewright, president and CEO of Walmart International, told staff: "Sean’s strong loyalty to Asda, where he began his career, was a key factor in accepting the challenge of repositioning our UK business in a very competitive market place.
"It’s clear from his time here our confidence was well founded. Sean has done a fantastic job during his tenure and we’re seeing a return to winning ways, including increased customer traffic and the start of market share growth.
"As always planned, Roger Burnley will make the intended transition as Asda’s new president and chief executive officer."
Mr Cheesewright said Mr Burnley has a long and distinguished retail career that includes multiple roles with Sainsbury’s and earlier time with Asda.
"Sean and Roger have partnered to define the direction for the business and have built a strong foundation for future success," the internal memo continued.
"We are confident in Roger’s ability to continue building upon our returning momentum."
Mr Cheesewright said that Mr Clarke will take "a well-deserved break" in January, but will remain with Walmart serving as an advisor.
Mr Burnley returned to Asda in October 2016 as chief operating officer, joining the board following Mr Clarke’s appointment as president and chief executive in July 2016.
At the time, Mr Cheesewright described Mr Clarke as “one of our most experienced global executives” whilst also recognising Roger as “a future CEO”.
Mr Burnley, said: “Asda is a great business and we’ve started to realise its potential again.
"Sean’s focus on serving customers and simplifying the business has established a firm foundation on which we can build. Since I returned to Asda last year, I have been encouraged by the passion and professionalism of our colleagues and look forward to leading this terrific team.”
Mr Clarke said: “Asda is moving in the right direction but there is still much more for us to do.
"I am excited to see the momentum continue under Roger’s leadership. Walmart has given me and my family some great opportunities. I am looking forward to taking some time out but I will always stay connected to the company.”