A rail company has begun an internal investigation after staff allegedly held a surprise baby shower in a closed Patisserie Valerie bakery at London's Marylebone station during lockdown.
Chiltern Railways workers told BBC News up to 20 colleagues, including some who were on shift, attended the gathering.
They claim some party-goers then had positive Covid tests, forcing most of the team to self-isolate.
Chiltern said "appropriate action" would be taken after its investigation.
Members of Chiltern Railways customer services staff based at the station told BBC News that about 30 people had been invited to the baby shower on the afternoon of 23 November - both via WhatsApp before the alleged gathering, and face to face on the day of the event.
A national coronavirus lockdown was in place in England in November, so people were banned from meeting anyone indoors who was not part of their household.
One worker, David [not his real name], said he declined an invitation to the event but walked past the bakery later in his shift to see about 20 colleagues gathered inside.
He said he was "shocked and alarmed" to see people hugging each other, with most of them not wearing masks.
Photos of the alleged gathering, seen by the BBC, show a table inside a Patisserie Valerie outlet covered with dozens of cupcakes, mince pies, crisps and sandwiches, bunting saying "it's a boy!" and handmade flags reading "happy baby shower".
One photo appears to show a group of eight colleagues posing in front of the table of party food, without socially distancing from one another.
Some images were shared on Instagram on 23 November with the caption: "What a lovely surprise being thrown a baby shower at work today!"
A Patisserie Valerie spokesman said the company had not been informed of any such event and that none of its team members had access to the Marylebone station cafe, which has remained closed since March due to Covid restrictions.
He added it was normal for a member of station staff to have keys to the premises for "security reasons".
David and another colleague claimed three people who allegedly attended the event tested positive over the following four days.
Covid cases 'nightmare'
The positive tests meant 16 members of staff out of the team of about 26 people had to self-isolate for 14 days, David said.
He said colleagues who lived with, or cared for, vulnerable people were "petrified" to hear there had been a staff outbreak, with some "scared to go home" for fear of endangering loved ones.
David added that he had been caring for his elderly grandmother so self-isolation was "a real nightmare" as he had to arrange alternative care for her.
Chiltern Railways confirmed a "small number" of workers tested positive for Covid or had to self-isolate in the 14-day period after 23 November, but a spokeswoman said "none of the staff who were alleged to have attended [the baby shower] tested positive".
She said Chiltern Railways was investigating and was "making every effort" to maintain a Covid-secure environment for staff and customers.
In an email seen by the BBC, which was sent to Chiltern Railways employees on 24 November, a manager said one team member had tested positive and added: "It is disappointing that social distancing measures do not appear to have been followed and I will be investigating this further."
David's colleague Peter (not his real name) said he was one of about 10 team members who had to work while the rest of the team was self-isolating.
Peter said the outbreak left those at work feeling "stretched" and "raised the anxiety levels of everyone" as they worried they might have caught Covid as a result of having worked alongside the alleged party's attendees.
"A lot of us don't want to be at work during this time, for obvious reasons. We're doing a job where we do come into contact with a lot of people - it's stressful enough with your own family, who are a bit worried about you going in to work at a train station and asking if you're getting the proper protection," Peter said.
He added he felt "demoralised" to hear about the alleged party when he spends his shifts encouraging customers to wear masks and socially distance.
The Department for Transport said it had been made aware of the incident and had contacted Chiltern Railways for a "full explanation".
A spokesman for the Office of Rail and Road - which protects the interests of rail and road users - said it had investigated "an issue relating to Covid-19 concerns" and had taken action, jointly with Westminster City Council, to "ensure Chiltern Railways tightens its risk assessment for workers and to revise working arrangements".