Covid restrictions are being toughened up in Parliament following concern over a surge in cases in Westminster.
Staff were told that the UK Health Security Agency (UKHSA) has ruled that the "risk of transmission on the parliamentary estate is now greater", forcing the authorities to act to prevent further increases.
And Commons Speaker Sir Lindsay Hoyle warned MPs that tougher measures could be coming if cases continue to rise.
All non-parliamentary business such as events, tours and banquets has been scrapped for the next two weeks, according to an email to staff, seen by the Mirror.
MPs are being asked to wear masks after many continued to snub face coverings in the packed out Commons chamber.
Masks were made mandatory for staffers, contractors, visitors and journalists last week, except when sitting down, eating and drinking, or speaking in a meeting.
MPs are now being asked to wear face coverings too in line with Government guidance, which states face coverings should be worn when the risk of transmission is greater.
But MPs are not employed by the Commons so they cannot be forced to obey the diktat.
Workers have been ordered to maintain distancing as far as possible in Parliament, with committee chairs told to be strict on enforcing the rules.
Face-to-face meetings "should be avoided, unless there is a business need", staff were told.
Sir Lindsay told MPs: "If we can get through these two weeks, I believe we're then through to next year.
"But it's about this crucial two weeks, as numbers have been rising on both sides of the House and within staff, and unusually the transmission has been on the estate and that's why it's a greater worry than we've had before."
He added: "I will always put the health and safety of this House first so please help me keep this House open by trying to get through a very crucial two weeks.
"After that I think we'll be in a much safer place, I think we'll be in the right place and the measures have not been stringent, they could have been even more stringent and some we might have to so please let us just pull and work together, because in the end I don't want to have another Christmas like we've had previously."
It comes after Commons leader Jacob Rees-Mogg recently claimed that Tory MPs didn't need to mask up because of their "convivial, fraternal spirit".
Amid increasing pressure to set an example, Boris Johnson finally donned a mask for the Budget last week, along with other top Tories such as Rishi Sunak and Sajid Javid.
Labour leader Keir Starmer was forced to miss the Budget and Prime Minister's Questions after testing positive for Covid moments before.
He is expected to miss PMQs tomorrow as he is still isolating. Deputy Leader Angela Rayner will fill in for him.
Shadow Cabinet ministers are understood to have held the regular Tuesday meeting on Zoom, rather than in person.
Some staff have been advised to work from home.
Labour MP Ben Bradshaw compared Parliament's contradictory rules to "Alice in Wonderland".
He tweeted: "You get chucked out of Parliament for not wearing a jacket & tie.
"If you won’t wear a mask, a recommended public health measure to protect others, nothing. What an antediluvian Alice in Wonderland this remains."
A Parliamentary spokesperson said: “The House’s priority is to ensure that those on the estate are safe while business is facilitated.
"There have been recent increases in COVID-19 across the country and these are also being reflected in Parliament. The UK Health Security Agency (UKHSA) has determined that the risk of transmission on the Parliamentary Estate is now greater.
"As a consequence, some further action is being taken to ensure that case numbers do not continue to rise. The measures will be reviewed in two weeks’ time.”
Downing Street said Boris Johnson would wear a mask in Parliament but it was a decision for every MP.
A No10 spokesman said: "It’s a matter for individuals to make that decision.
"You can expect to see the same from him again, based on what he did last week in the house."
Asked if the PM was disappointed in the Commons taking a tough line, the spokesman said: "You've heard the Prime Minister speak before about the ultimate winter period and the fact that we know that it's going to be difficult we set out quite clearly in our autumn winter plan, what plan A and plan B is and you'll be aware that we are not telling people what they need to do and plan a relies heavily on people's own decisions.
"What Parliament choose to do in the parliamentary state as I say, is a matter for the parliamentary authorities."
