Germany could see 100,000 infections a day if the third wave of coronavirus spreads unchecked, the head of the RKI public health institute has warned.
Fears that the third Covid wave could be the worst so far have prompted a clampdown on borders.
Negative tests will be required for airport arrivals from Tuesday and all of France has been declared high-risk.
From Sunday anyone travelling from France will have to submit a negative test and go into 10 days' quarantine.
Random German checks and compulsory tests will be enforced on the French border, says France's foreign minister. "The pandemic in Germany is exploding faster than they thought," said Jean-Yves Le Drian.
With infections running at more than 20,000 a day, German Health Minister Jens Spahn warned: "If this continues unchecked we run the risk of our health system hitting breaking point in April."