“What we have to do is take swift and decisive action where we think that the risks are starting to bubble up again,” Johnson told media during a visit to Nottinghamshire.
“Let’s be absolutely clear about what’s happening in Europe, amongst some of our European friends, I’m afraid you are starting to see in some places the signs of a second wave of the pandemic.”
“And we all remember what happened last time. It’s absolutely vital, therefore, that we make the necessary preparations here in the UK, as we are doing,” he said.
Britain abruptly announced late Saturday that it had taken Spain off a safe-travel list, imposing a 14-day quarantine on anyone travelling from that country.
On Monday, the UK extended its travel advisory against travel in mainland Spain to the Canary and Balearic islands.
Since ending its nationwide lockdown a month ago, Spain has been grappling with a rapid proliferation of new cases.
The health ministry reported 6,361 new cases over the weekend and said it was monitoring 361 clusters around the country.
But Spanish Prime Minister Pedro Sanchez called the British decision an “error.”
The rebound in infections is focused in two regions, Catalonia and Aragon, Sanchez told Telecinco television on Monday.
“In most of Spain, the incidence [of the disease] is very much inferior to even the numbers registered in the UK,” he said.
Johnson defended the UK’s decision on Tuesday, saying, “I’m afraid if we do see signs of a second wave in other countries it is really our job, our duty, to act swiftly and decisively to stop travellers coming back from those places seeding the disease here in the UK.”
Elsewhere in Europe, a series of localized outbreaks forced the Belgian government to tighten virus containment measures last Saturday, requiring the use of masks in crowded outdoor public spaces and tracing measures at restaurants and bars.
Greece announced new measures on Tuesday, requiring masks to be worn inside retail shops, offices, banks, food stores, and bakeries, though churches are excluded for the time being.
In Austria, 62 people tested positive in the lakeside resort town of St. Wolfgang after an outbreak was detected on July 24.
New infections have also been creeping higher in Germany, where the government made COVID-19 tests at airports mandatory for anyone returning from high-risk areas.
Reuters contributed to this report.