More than 500 people are self-isolating because of a coronavirus outbreak following a posh hotel party.
The "super-spreader" bash at the luxurious Royal Yacht Hotel, on Jersey, is now responsible for a quarter of all cases on the Channel Island, authorities have confirmed.
Sources say those currently in isolation as result include direct contacts, whole school year groups and their families.
More than 2,000 people are self-isolating across Jersey at present, meaning a quarter of all isolations are directly linked to the party at the four-star St Helier hotel.
The government said it was continuing to investigate whether any rules were broken by the gathering but the hotel manager has said the event and all attendees followed government guidelines.
Senator John Le Fondré, Jersey's Chief Minister, said he was "extremely disappointed" by the news.
He said: "As we can see, one event like this can have a significant impact on our statistics.
"The fact we can get this early means hopefully we can isolate that cluster, and again we're managing the risk.
"But it all comes down to that point of us all being vigilant, of acting responsibly, if we really want to ensure that we're going to avoid a lockdown in the run up to Christmas."
A spokesperson for the hotel told ITV News: "Mr Vieira, the Hotel General Manager, can confirm that one member of staff is currently in isolation after being contacted directly by the track and trace team.
"Their test has since come back negative. As with any other establishment/business on island, we are unable to comment about individual events or persons.
"The hotel has a duty of care to protect the privacy and confidentiality of all those who frequent the hotel, whilst also prioritising their health and wellbeing. We will continue to do this as best we can.
"The Royal Yacht Hotel is aware of all local guidelines and legislation and we will continue to work within these parameters. We cannot comment further at this time.”
The Government of Jersey says the event is being investigated to check if those who attended followed public health rules.
Current guidelines, which are due to become enforceable next week, limit social gatherings on the island to ten people or less with up to 20 people able to attend officially organised events.
In Jersey three parties have been made responsible for the vast majority of isolations.
There were a total of 34 "clusters" discovered in November, ranging from incidents in homes to parties in hotels, as well as outbreaks connected to bars and restaurants.
Dr Ivan Muscat confirmed that, on average, each person testing positive in a cluster case leads to 10-15 other people being tracked down and put in isolation.
A second event led to 630 people being contacted, with a third leading to 530 such cases.
The Manager of Jersey's Royal Yacht Hotel has hit back at criticism following a private party held there last week.
Alcino Vieira said: "I find it difficult to understand why the Royal Yacht has been singled out given that a number of other significant clusters have occurred throughout November in both the hospitality and the non-hospitality sectors.
"The event, and all those who attended it, followed all government guidelines.
"We are concerned that the reaction to this particular cluster, and the vilification of certain venues and individuals in the media and on social media, may deter others from taking the correct course of action and thereby undermining the viability and the effectiveness of the systems in place."
He added: "We will continue to strive to provide a safe and enjoyable environment for all of our guests".