Belgium announced new rules Monday as it tries to stem a worrying flare-up in coronavirus cases centered on the port city of Antwerp.
From Wednesday, Belgians will be allowed to see a maximum of five people outside of their families, reducing the permitted "social bubble" from 15.
Working from home will be "strongly recommended."
The announcement from Prime Minister Sophie Wilmes comes as fears grow in some European countries of a damaging second wave of the pandemic, with Britain reimposing quarantine requirements on holidaymakers returning from Spain.
Belgian health officials said there had been a rapid rise in coronavirus cases, with 1,952 new infections over the past week, up more than 70 percent on the previous week.
"The epidemiological data are worrying and we are very worried, that's why we are acting today," Wilmes told reporters, after a meeting of the country's national security council.
In addition, face masks are to become compulsory whenever people are unable to maintain a social distance of 1.5 meters, and the government is "strongly recommending" that people work from home.
The moves mark a sharp turnaround after Belgium began to ease anti-virus measures, but Wilmes said it was needed "to avoid a new complete lockdown and to avoid putting the return of schools in September in jeopardy".
Frederique Jacobs, professor of infectious diseases and a spokeswoman for Belgium's federal COVID-19 taskforce, told reporters: "The rapid growth in the number of cases is worrying."
She said the situation was particularly serious in the province of Antwerp, in northern Belgium, which recorded 47 percent of new infections in the last week.
Special local measures have been taken to try to contain the spike.
But Jacobs warned that "the number of infections is also rising considerably in the rest of the country".
An average of 279 people a day contracted COVID-19 in the last week, compared with 163 a day the week before.
"Most infections are still coming among the active population, that is to say people aged between 20 and 59," Jacobs said.
Since the start of the pandemic Belgium has recorded 66,026 cases and 9,821 deaths, according to the latest data published on Monday, making it one of the countries with the highest per capita rates of infection at the height of crisis in Europe.
A three-year-old girl with severe pre-existing health problems became Belgium's youngest known coronavirus victim last week.
Tougher rules on wearing masks came into force on Saturday with face coverings made obligatory in markets, hotels and busy shopping streets.
Meanwhile, travel giant TUI cancelled all British holidays to mainland Spain from Monday after London announced that all travellers returning from the country would have to go into two week's quarantine.
The newly-imposed rule, abruptly introduced at midnight Saturday hours after being announced, follows a surge in coronavirus cases in parts of Spain in recent weeks.
The quarantine rule has dismayed the tourism sector, which was aiming to recover some of its revenues during the peak summer weeks after being decimated by months of lockdown.
There was further unsettling news from Britain where health officials said a pet cat had become the country's first animal to be confirmed with COVID-19.
The cat was initially diagnosed by a private vet as having the feline herpes virus but the sample was also subsequently tested for COVID-19 and proved positive.
Officials said it was a very rare event and there was no evidence that pets transmit the virus directly to humans.