Covid: Masks mandatory for everyone in the Commons - except MPs
In updated guidance, the Commons authorities said all staff, visitors, contractors and press must cover their faces to combat the spread of Covid.
But it remains up to individual MPs to decide whether to follow suit - and many Conservatives have chosen not to.
Sajid Javid has said he will wear a mask for Wednesday's Budget when the chamber will be packed.
But the health secretary said on Monday it was a "personal decision" for ministers and backbenchers as to whether they did too.
MPs are not employed by the Commons authorities and cannot be forced to wear masks.
They have been encouraged to do so by Speaker Sir Lindsay Hoyle - but unions representing Parliamentary workers have urged him to take a tougher line.
'The virus doesn't vote'
Most opposition parties, including Labour, the Lib Dems and the SNP have decided to cover their faces during debates.
But Commons Leader Jacob Rees-Mogg last week said Conservatives did not need to do so because they knew each other well, and this meant they were complying with government guidance.
And he claimed Labour MPs only wore masks when the television cameras were around.
The latest official guidance says people in England should cover their faces around "people you don't normally meet".
It comes as the World Health Organisation (WHO) urged MPs to wear masks during Chancellor Rishi Sunak's Budget speech, when there is likely to be standing room only in the Commons chamber.
Dr David Nabarro, the WHO's special envoy for Covid-19, told Sky News that "everybody" should be wearing masks in close confinement with other people, "including our leaders".
"This virus, it is absolutely unstoppable, it gets everywhere, and so we have to do everything we possibly can to stop it.
"And one of the best ways to stop it is a well-fitting surgical mask properly over your face, pushed in over your nose, covering everything, and that reduces the risk to others and the risk to you.
"If it works, why on earth don't people use it? It's not a party political issue - this virus doesn't vote."