The chain, with pubs across Norfolk, said bar and food sales are down 16.9% for the 44 days to August 16, compared to last year.
The group has reopened 844 of its 873 pubs, after shutting all sites in March as a result of coronavirus.
Tim Martin, founder of Wetherspoon, said the sales slump means it expects to post a loss for the year to July 26.
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“Sales have gradually improved, with a rapid acceleration recently, largely due to subsidised food, coffee and soft drinks in the early part of the week,” the company said in a statement.
It said sales had also been boosted by an increase in outdoor seating, while it had also benefited from “extremely flexible” landlords, local authorities and licensing authorities.
A member of staff at Wetherspoon’s pub in Gorleston, the William Adams, tested positive for coronavirus recently. Mr Martin said: “Wetherspoon had five positive tests for Covid-19 among its 43,000 staff before lockdown and has had 24 positive tests since pubs reopened on July 4.”
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