Belgium’s prime minister Alexander De Croo announced the measure on Tuesday, telling reporters that the decision had been made because of a 70% increase in cases.
Remote working is also being encouraged in an effort to tackle what De Croo described as an “autumn wave”.
He said: “The consultative committee met early today. Nobody can ignore the figures. In our country and other countries, you see an autumn wave after a plateau.
“Last week we saw a 70% increase in terms of infections.”
The new rule will come into place on Friday, and people will have to cover their faces in shops, shopping centres, healthcare institutions, concert halls, sports centres, libraries and places of worship.
However, masks will not be need in places that accept Belgium’s version of a vaccine passport – known as the COVID Safe Ticket (CST).
De Croo insisted that vaccines are the “most important weapon against this virus”.
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He added: “We need to build higher walls of protection, which ensure that our lives still go on.
“A free, but cautious life. We must be careful about the number of contacts we have.”
Last week, the NHS Confederation and the British Medical Association (BMA) called on UK ministers to activate their winter Plan B for England amid fears the health service could be overwhelmed.
BMA council chair Dr Chaand Nagpaul said the refusal to introduce supplementary measures – including COVID passports, mask-wearing in crowded public spaces and a return to working from home – amounted to “wilful negligence”.
But Boris Johnson rejected the calls, acknowledging that COVID numbers were “high” but “within the parameters” forecast by scientists advising the government.
However, the House of Commons ordered everyone to wear face coverings while on the parliamentary estate from this week – except MPs.
Health secretary Sajid Javid said on Monday wearing a mask in the crowded chamber is a “personal decision” for ministers and backbenchers, while Commons Leader Jacob Rees-Mogg last week insisted Tories do not need to wear masks in parliament because with their “convivial, fraternal spirit” they know each other well.
But some members of the government chose to wear masks on Wednesday for the chancellor’s budget.
While the prime minister wore a mask alongside other cabinet members, notable exceptions included Rees-Mogg and foreign secretary Liz Truss.
There was also a clear difference on face coverings across the chamber, with universal mask wearing on the opposition benches.
By contrast, there were more Tory MPs without masks in the packed chamber than those with.
The UK reported 43,941 coronavirus cases on Wednesday, with the total down 4% from the previous seven days. It also reported 207 deaths.
Watch: Health secretary defends government stance on COVID masks