Germany's Foreign Ministry said it had toughened up its warning against travel to the regions of Catalonia, Navarre and Aragon following a spike in Covid-19 cases there.
The move comes after Germany's Robert Koch Institute (RKI) for disease control added the three regions to its high-risk list.
“A mandatory quarantine can only be avoided through a negative Covid-19 test,” the ministry said.
The Ministry had on Tuesday already advised against non-essential travel to the three regions but Friday's tightened restrictions underline the growing alarm about returning holidaymakers bringing the virus back with them.
The affected regions include the tourist hotspots of Barcelona and the beaches of the Costa Brava.
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Germany, itself grappling with a surge in cases, has announced mandatory, free coronavirus tests at airports for anyone entering the country from a high-risk area.
German Health Minister Jens Spahn on Friday said he expected the scheme to be up and running next week.
Travellers returning from non-risk areas who wish to get tested will also have free access to tests within 72 hours of arriving back in Germany, he said.
“The rising infection numbers in Germany are a clear warning signal. The virus doesn't take a vacation,” he said.
Germany on Friday reported an additional 870 coronavirus cases, bringing the total so far to 208,698.
Of those, 9,141 people have died from the virus so far.
RKI head Lothar Wieler this week voiced “great concern” over the uptick in cases, which comes as countries across Europe are battling a surge in infections.