Shares in Ocado Group fell sharply on Friday, after key US partner Kroger flagged a possible change in direction.
Investors were reacting to comments made by Kroger - the UK firm's biggest partner for its warehouse technology - during a second-quarter earnings call on Thursday.
Kroger, which is carrying out a strategic review, told analysts it would take a "hard look" at some of its automated facilities and carry out a full, site-by-site analysis of its existing network.
It also noted it could offer delivery in under two hours from 97% of its shops, and flagged its "conveniently located store network".
As at 0930 BST, shares in Ocado were topping the FTSE 250 fallers board, down 11% at 268.4p.
Neil Wilson, UK investor strategist at Saxo Markets, said: "The comments are clearly a negative for Ocado as Kroger seems likely to move away from the kind of large customer fulfilment centres provided by the British company, and instead seems to be looking to lean on local stores to fill orders."
Morgan Stanley agreed, noting the "negative readacross" from Kroger's comments for Ocado.
According to Bloomberg, the bank pointed to an "increasing risk" that some of Ocado's existing sites with Kroger could be closed, while the heightened focus on store delivery put "a question mark on where the long-term CFC network fits in".
As well as delivering groceries in the UK in partnership with Marks & Spencer, Ocado sells technology and logistics services to retailers worldwide.
In a series of social media posts on Thursday, Ocado flagged Kroger's strong e-commerce sales during the quarter. It also quoted chief executive Ron Sargent, who said e-commerce remained a "top priority" for the retailer.