(Bloomberg) -- Ocado Group Plc shares plunged the most in 11 years after a top-ranked analyst and former long-term bear resumed his negative view, just three months after upgrading the UK online grocer.

The stock closed down 20%, having briefly been halted earlier due to volatility. BNP Paribas Exane’s Andrew Gwynn downgraded his recommendation to underperform from neutral, saying a recent rally had left the risk versus reward “out of kilter.”

Writing in a note, Gwynn also cited subdued retail volumes and added that the retirement of Ocado Solutions boss Luke Jensen “does not point to an over-flowing order book” in the robotic warehouse unit.

A spokesperson for Ocado declined to comment.

Since Exane upgraded the stock in June, Ocado shares have been boosted by bid speculation and the settlement of a dispute over patent infringements regarding automated warehouses. Prior to that, Exane had a sell-equivalent rating for five years, according to data compiled by Bloomberg.

Even after Thursday’s stock plunge, Ocado remains the best performer in Europe’s benchmark Stoxx 600 Index this quarter. Shares out on loan, an indication of short interest, represented about 11% of Ocado’s free float as of Wednesday, down from a 12-month high of about 18% in April, according to data from S&P Global Market Intelligence.

--With assistance from Thyagaraju Adinarayan.

(Updates to include Ocado declining to comment in fourth paragraph.)

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