LONDON — Prime Minister Boris Johnson was moved into intensive care on Monday, a worrisome turn in his 10-day battle with the coronavirus and the starkest evidence yet of how the virus has threatened the British political establishment and thrown its new government into upheaval.

The government said that the decision was a precaution and that he had been in good spirits earlier in the day. But with Johnson’s aides releasing few details about his condition, the nation kept a tense vigil Monday night, hoping for the best and experiencing, together, the frightening mysteries of this disease.

In a sign of how grave the situation had become, No. 10 Downing St. said in a statement Monday that Johnson had asked the foreign secretary, Dominic Raab, to deputize for him “where necessary.” The pound fell against the dollar after investors reacted to the news.

After noting earlier in the day that the prime minister was still getting official papers, Johnson’s aides said he had been moved to the intensive care unit in case he needed a ventilator to help his recovery. Not every patient in critical care is ventilated, medical experts said, but many are — or are at least given oxygen.

For Johnson, 55, it was an especially cruel reversal. Just four months ago, he engineered the greatest Conservative Party victory since Margaret Thatcher in 1987, delivered his promise to take Britain out of the European Union, and set in motion an ambitious economic program to transform his divided country.

British politicians from across the political spectrum offered their best wishes, as did the French president, Emmanuel Macron, who said he hoped Johnson would make a swift recovery. Buckingham Palace said Downing Street had informed the queen of Johnson’s condition.

On Monday, at his daily briefing, President Donald Trump wished Johnson well. “Americans are all praying for him,” he said. “He’s been a really good friend.” The president suggested that American pharmaceutical companies could aid the prime minister by supplying therapeutic drugs, although there are not yet any proven treatments. He also said he and other senior officials would take more tests for the virus in light of Johnson’s ordeal.