Twitter has told employees that the company’s office buildings will be temporarily closed, with immediate effect.
In a message seen by the BBC, workers were told that the offices would reopen on Monday 21 November.
It did not give a reason for the move.
The announcement comes amid reports that large numbers of staff were quitting after new owner Elon Musk called on them to sign up for “long hours at high intensity” or leave.
The message went on to say: “Please continue to comply with company policy by refraining from discussing confidential company information on social media, with the press or elsewhere.”
The reports have been met with consternation from unions with Prospect, the union for tech workers, asking Twitter UK to meet regarding the treatment of its employees.
“We will not let these makings of a digital P&O pass unchecked,” said Mike Clancy, general secretary of Prospect, referring to the ferry operator’s decision to sack staff and replace them with agency workers earlier this year.
“We are urgently seeking a meeting with Twitter UK Ltd to discuss how it will manage its collective redundancy consultation, ensure a fair and transparent process, and meet its duty of care and legal obligations to employees, including those with particular needs.
“Prospect will continue to do everything we can to support our members at Twitter. Big tech barons are not above the law and we will hold Twitter to legal account where possible.”
‘Nobody left in chain of command’
There are signs that large numbers of workers have resigned because they have not accepted Mr Musk’s new terms.
One former Twitter employee, who wished to remain anonymous, told the BBC: “I think when the dust clears today, there’s probably going to be less than 2,000 people left.”
They claimed everyone in their team had been sacked.
“The manager of that team, his manager was terminated. And then that manager’s manager was terminated. The person above that was one of the execs terminated on the first day. So there’s nobody left in that chain of command.”
Another person said they had resigned even though they had been prepared to work long hours.
“I didn’t want to work for someone who threatened us over email multiple times about only ‘exceptional tweeps should work here’ when I was already working 60-70 hours weekly,” they said.
Twitter did not immediately respond to a request for comment from the BBC.
In response to staff leaving, former Twitter vice-president Bruce Daisley told the BBC there were former Twitter engineers claiming the social media platform could “fail as soon as Monday”.
“There’s a large number of features that really seem to be predicated on having engineers on site,” he said.
“If those engineers have gone, then it does threaten the sustainability of the product.
“So, there’s a lot of people posting where else you can find them online.”
This week, Mr Musk told Twitter staff that they had to commit to working long hours and would “need to be extremely hardcore” or leave the company.
In an email to staff, the firm’s new owner said workers should agree to the pledge if they wanted to stay, the Washington Post reported.
Those who did not sign up by Thursday 17 November would be given three months’ severance pay, Mr Musk said.
Earlier this month the company said that it was cutting about 50% of its workforce.
Employees have been tweeting using the hashtag #LoveWhereYouWorked and a saluting emoji to show they were leaving the firm.
Despite the turmoil at the company, Mr Musk tweeted on Friday: “And… we just hit another all-time high in Twitter usage lol.”
Satirist Frank Lesser replied: “‘Rome has never been this brightly lit at night!’ – Nero” – a reference to the Roman emperor who is reputed to have fiddled while Rome burned.
Before Mr Musk took control of Twitter the company had about 7,500 staff. The firm was also reported to have employed thousands of contract workers, the majority of whom are understood to have been laid off.
The world’s richest person became Twitter’s chief executive after buying the firm last month in a $44bn (£37bn) deal.
Mr Musk seemed unconcerned about reports that Twitter was on the brink of shutting down, tweeting: “The best people are staying, so I’m not super worried”.
In separate posts he tweeted a skull and crossbones emoji and a meme showing a gravestone with the Twitter logo on it.