A third of the sailors serving with France's aircraft carrier Charles de Gaulle – 668 out of nearly 2,000 – are infected with coronavirus.
Nearly all are on the carrier itself, the navy says. An escorting frigate and carrier pilots are also in quarantine.
The carrier returned to the French port of Toulon early from Atlantic exercises. Twenty sailors are in hospital, one in intensive care.
The infection total looks set to rise, as 30% of test results are not yet in.
The navy is investigating how so many sailors caught the virus. Last week the aircraft carrier was brought home 10 days early from its Atlantic deployment after some sailors showed symptoms.
Earlier this month nearly 600 coronavirus cases were confirmed aboard the USS Theodore Roosevelt, one of two US aircraft carriers in the western Pacific.
One sailor later died of Covid-19 in Guam, after the ship – which has a total crew of 4,800 – had docked there.
The ship's captain Brett Crozier was fired after his letter pleading for help with the outbreak was leaked to US media. A public outcry over that dismissal triggered the resignation of acting Navy secretary Thomas Modly.
Meanwhile, a Dutch navy submarine, MS Dolfijn, has returned to its Den Helder base two weeks early because of a coronavirus outbreak on board.
Eight of the 58 crew tested positive and the submarine, which had been sailing near Scotland, is now in quarantine.
Pandemic strikes naval deployments
Proximity and contagion make for dangerous shipmates. We have already seen how deadly the virus can be in the close confines of civilian cruise ships, albeit with many older passengers. Now France's only carrier and its naval flagship, the Charles de Gaulle, has been hit.
Following the outbreak on the US carrier USS Theodore Roosevelt, the US Navy is attempting to quarantine the crew of the USS Nimitz on board, ahead of a future deployment. The problems of confinement are heightened on such large vessels.
A typical US nuclear-powered carrier has a crew of over 5,000, including the ship's sailors and air wing. Aircraft carriers are the prime tool of global military presence and this pandemic is proving hugely problematic for the US Navy.
The number of carriers available is limited and there are never enough. The US Navy is taking drastic measures. The carrier USS Harry S. Truman, at the end of a seven-month mission, is being held offshore, to ensure that at least one carrier is ready for a short-notice surge deployment if required.
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France's Covid-19 death toll is 17,167 while 10,643 of those died in hospital. Health authorities said on Wednesday 6,457 Covid-19 patients were in intensive care, 273 fewer than on Tuesday.
The country has a very strict lockdown, which President Emmanuel Macron has extended to 11 May. It has the third-highest death toll in Europe after Italy and Spain, however some European countries appear to be under-reporting care home deaths.