The companies released a short statement on Tuesday, saying there was no guarantee a deal would emerge, after London's Evening Standard newspaper said Marks & Spencer (M&S) was set to pay up to $1.2 billion for a 50 percent stake in a joint venture with Ocado.
Ocado, founded by three Goldman Sachs bankers 18 years ago, has been transformed in the last year after it struck major deals to sell its technology to international retailers such as U.S. group Kroger Co, France's Casino.
The Evening Standard report suggested Ocado would use M&S as a supply partner for British customers. It was not clear what this would mean for its current partnership with the John Lewis-owned Waitrose supermarket chain.
Shares in Ocado were up 10 percent at 1142 GMT, while M&S was up 4 percent.