The most successful investor in the world has ditched nearly all of his shares in Walmart and upped his bet on Apple.

Warren Buffett has just sold $900 million (£720 million) worth of Walmart, the owner of Asda in the UK, and pumped the money into the iPhones maker and airlines stocks.

The switch will be noted with concern in retail circles given Buffett’s influence and track record, with some seeing the move as an admission Amazon cannot be stopped.

Buffett, the third-richest man in the world, famously bought Tesco back in 2006, at one point having a 5% stake in the supermarket. He later admitted this had been a “huge mistake” and sold the £1.5 billion holding at a big loss.

Regulatory filings in the US show his Berkshire Hathaway holding company quadrupled its stake in Apple from 15.2 million shares to 57.4 million. That stake is worth towards $8 billion, with Apple shares at record highs.

Amazon’s market value is $356 billion, compared with Walmart’s $298 billion. Last year, Buffett acknowledged traditional brick-and-mortar retailers were struggling in the face of competition from the e-commerce giant.

“It is a big, big force and it has already disrupted plenty of people and it will disrupt more,” Buffett said at his annual shareholders’ meeting in 2016.

The so-called Sage of Omaha is the subject of an HBO documentary about his life called Becoming Warren Buffett. “I like numbers,” he tells the film-makers.