The pub group, which has reopened 844 of its 873 sites, said bar and food sales fell by 16.9pc for the 44 days to 16 August.
Wetherspoon said sales had seen a “rapid acceleration” following the launch of Rishi Sunak’s discount dining scheme that halves the cost of food and soft drinks up to a maximum of £10 per person from Monday to Wednesday in August.
Sales have also improved as a result of additional seating outside its pubs, the company added.
However, it warned that sales are expected to be more subdued once the Chancellor’s scheme ends. Chairman Tim Martin said it expects to fall to a loss for the year to 26 July due to one-off costs related to the pandemic.
Wetherspoon tapped shareholders for £141m in April and secured £48.3m through the Government’s coronavirus business interruption loan scheme to help it weather the pandemic.
It also secured waivers from its banks for lending arrangements in April and July and is preparing to enter discussions with lenders for waivers for the current financial year.
Mr Martin has spoken out against some of the UK’s leading scientists after they warned that drinking in pubs was tied to localised outbreaks of the virus.
Alongside the trading update, he argued that compared to Wetherspoon’s pubs, other industries had seen higher levels of infection.
"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 4 July - since reopening, the amount of testing has substantially increased," Mr Martin said.
"Other environments seem to have higher levels of infection. For example, one sandwich-making facility in Northampton had 287 positive tests among its workforce, and one farm in Hereford had 77 cases.
"Risk cannot be eliminated completely in pubs, but sensible social distancing and hygiene policies, combined with continued assistance and cooperation from the authorities, should minimise it."
Shares rose 1.1pc to 984p. The stock was close to £17 at the start of the year.