At least 30 staff at a Wetherspoon branch in Liverpool are self-isolating after 12 tested positive for Covid-19.
The firm has contacted Public Health England to make the organisation aware of the outbreak among staff at the Lime Kiln, in Concert Square.
Pub bosses told the ECHO they had received no reports of transmission between staff and customers and insisted a "rigorous implementation of hygiene standards" was in place at the venue, which remains open.
A spokesman said they believe a "private event" attended by some members of staff outside work may be linked to the cases.
The Lime Kiln was told of its first two coronavirus cases among staff on September 16, leading to the affected employees and 20 others thought to have been in close contact with them being told to self-isolate.
A further 10 positive cases among staff were then identified on September 18 and 19, leading to more employees being told to self-isolate.
A spokesman for JD Wetherspoon said Public Health England had been told of the initial cases, would be updated about the rest and that the pub firm would work with health bosses if any measures are recommended.
NHS Test and Trace have contacted the pub to request details of the staff who have tested positive and that information has been provided, the ECHO was told.
The spokesman said: "A number employees attended a private event outside of work, which, we believe, may be linked to the cases. At the pub, social distancing rules are in place, which minimises risk of transmission.
"We appreciate that there may be some concerns from customers, but the company has not been informed of any cases of infections being transmitted from staff to customers at the pub.
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"We understand that all the employees who tested positive have experienced mild symptoms and we wish them well.
"Since reopening, Wetherspoon has operated comprehensive social distancing and hygiene practices in all its pubs.
"These include reduced capacity levels, the spacing out of tables, the installation of a number of floor screens between tables and the addition of till-surround screens to the bar.
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"Staff are conducting regular surface cleaning and numerous hand sanitisers have been installed in each pub.
"While it is not possible to create an entirely risk-free environment, adherence to social distancing guidelines and rigorous implementation of hygiene standards minimises the risk."
