Ocado risks a customer exodus as a result of its tie-up with Marks & Spencer after a straw poll by analysts suggested shoppers were loyal to Waitrose and did not see M&S products as an “adequate” replacement.

On Friday, a note by HSBC’s retail analyst David McCarthy revealed the opinions of 250 Ocado shoppers on the deal, views which he described as “worrying” for the two listed companies.

Just over a fifth said they would not shop with Ocado if Waitrose products were not available. A “meaningful proportion” said their loyalty was to Waitrose and that M&S was not an adequate replacement.

M&S is buying half of Ocado’s UK retail business for £750m in a deal that will result in its food replacing Waitrose products on the Ocado website in 2020. Ocado has played down the risk of losing customers from its 720,000-strong base after the brands switch over. But McCarthy warned heavy customer losses would hit the bottom line if his straw poll turned out to be representative of that base.

“The profit implications could be severe due to operational gearing and low profits,” he wrote. “If the joint venture lost sales … losses could rise sharply.”

Waitrose says roughly 40% of Ocado shoppers visit its stores, with the HSBC survey showing that Waitrose’s products were more important to the respondents than Ocado’s.

Facebook Twitter Pinterest Ocado’s chief executive, Tim Steiner, left, and his M&S counterpart, Steve Rowe, announce the joint venture in February. Photograph: REX/Shutterstock

The supply deal with Ocado was worth about £15m a year in fee income for Waitrose, which may have to work hard to convince shoppers to switch to its own delivery service. Waitrose.com fills shoppers’ orders from its stores’ shelves, as well as a fulfilment centre in Coulsdon, south London.

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But Ocado is not wasting any time either. On Friday it wrote to customers trumpeting the M&S deal – promising the “only change you’ll notice” come September 2020 is that “you’ll be able to buy thousands of M&S own-label groceries from Ocado”. “Together with M&S we will develop new and exciting lines, resulting in an even bigger range,” it added.

After last month’s deal, analysts suggested M&S was overpaying for the Ocado stake, which is being financed via a £600m rights issue and a cut to its dividend. The chief executive of M&S, Steve Rowe, has said the deal was a good one for the retailer, which has previously struggled to make the economics of selling its upmarket food hall ranges online. “This is not about the short term,” he said. “This is about the transformation of M&S [and] the transformation of online grocery shopping in the UK.”

In his note, McCarthy concluded: “Ocado has done a good deal, as its lack of buying power … and its reliance on Waitrose was a major strategic threat. Of course consumers do not always do what they say they will do when faced with future events, and M&S will expand its range, but our survey suggests the challenge for the joint venture may be bigger than management expects.”