Nicola Sturgeon has hit back at suggestions that a coronavrius outbreak at a conference in Edinburgh in February was covered up.
At least 25 people linked to the Nike event may have contracted the virus but people who shared facilities with delegates were not told.
The first minister said details were not made public at the time because of patient confidentiality guidelines.
But MP Ian Murray accused Ms Surgeon of keeping “vital information secret”.
BBC Scotland has been told that staff from Lloyds Banking Group were among those who shared facilities with Nike delegates.
A total of 20 Lloyds employees were in the same hotel on the same day but were not told about the outbreak and were not offered testing.
This comes after The Mail on Sunday reported that staff at two other firms fell ill after coming into contact with Nike delegates.
The newspaper said the businesses – a marketing firm and a kilt hire company – were not contacted at the time of the outbreak.
The conference has been described as Scotland’s “ground zero”, where community transmission first happened.
No ‘cover up’ says Sturgeon
Ms Sturgeon insisted all proper procedures were followed and “contact tracing was done rigorously in this situation”.
Speaking at the daily government briefing on Monday, the first minister said she would “caution people against latching on to emotive phrases like ‘covering things up'”.
She added: “I would simply ask people what on earth would have been the motive for not putting more information into the public domain should it have been possible to do so?”.
But Ms Sturgeon’s explanation that patient confidentiality was paramount in this situation is wrong, according to shadow Scottish Secretary Ian Murray.
The Edinburgh South MP has written to the first minister to claim the patient confidentiality point is “unfounded in terms of law and runs counter to the recognition in law of the over-riding public interest”.
Mr Murray says the Public Health (Scotland) Act 2008 supersedes all other considerations if “it is necessary to do so for the purposes of, or in connection with, the protection of public health”.
The Labour MP added: “The cover-up of this early and significant outbreak has shattered public confidence in the Scottish government.
“Questions are piling up for Nicola Sturgeon about her decision to keep this vital information secret, which we now know had consequences far and wide.
“There is not the slightest doubt that sharing information about the Edinburgh outbreak would have transformed public perceptions, safeguarded my constituents and led nationally to an acceleration of counter-measures.”
The Scottish government said all of the Nike cases that were later confirmed were included in the daily totals published and Health Protection Scotland had established an incident management team at the time.
Nike told the BBC it had instigated enhanced measures, including contact tracing and increased cleaning and disinfection processes in its stores and offices, after the outbreak at its Edinburgh conference, and said that all of its staff had now recovered.