Queen Elizabeth II spent a night in the hospital for tests after being forced to cancel a visit to Northern Ireland this week, Buckingham Palace said Thursday.
"Following medical advice to rest for a few days, The Queen attended hospital on Wednesday afternoon for some preliminary investigations, returning to Windsor Castle at lunchtime today, and remains in good spirits," a palace statement said.
Britain's domestic Press Association news agency said the trip to the hospital was unannounced as it was expected to be a short stay, and also to protect the 95-year-old monarch's privacy.
The overnight stay was for "practical reasons," it added.
Elizabeth was seen by specialists at the private King Edward VII's Hospital in central London, where her late husband, Prince Philip, spent four weeks from February this year for treatment for a preexisting heart condition.
Philip, who was married to the queen for 73 years, died in April just a few weeks before his 100th birthday.
The queen, who has been on the throne since 1952 and is Britain's longest-serving monarch, was said to be back at her desk on Thursday afternoon, undertaking light duties.
She had been due to attend an ecumenical service in the border town of Armagh on Thursday to mark the 100th centenary of the creation of Northern Ireland.
But the palace said on Wednesday morning that she had "reluctantly accepted medical advice to rest for the next few days."
The decision was not related to the coronavirus, and she was said to have been resting at her Windsor Castle residence, west of London.
The queen has had a busy schedule since returning from her remote Balmoral estate in northeast Scotland at the start of October.
She has resumed public engagements since the funeral of Prince Philip, either alone or accompanied by other senior royals.
Last week, she delivered a speech at the opening of the Welsh Assembly in Cardiff, and on the weekend spent a day at Ascot Racecourse.
On Monday, she held a virtual audience with the new governor-general of New Zealand, and on Tuesday received two ambassadors, also by video link.
On Tuesday evening, she hosted a reception at Windsor for international business leaders attending a government investment summit, including the billionaire philanthropist Bill Gates, and senior British ministers.
At that reception, the queen appeared cheerful as she, her eldest son and heir Prince Charles, 72, and grandson Prince William, 39, mingled with guests, none of whom were wearing face masks.
Coronavirus restrictions were lifted in Britain in July, but an increase in cases has prompted calls for measures to be reimposed to prevent further close-contact transmission.
Daily virus cases crossed the 50,000 mark on Thursday, according to the latest government figures, the highest since July 17.
Elizabeth and Philip moved to Windsor in March last year as the coronavirus outbreak took hold.
They decided to self-isolate because of the increased risk of infection due to their age, although she has since been vaccinated.
The queen is still expected to join other senior royals for a series of events linked to the upcoming United Nations climate summit in Glasgow next month.
She was seen last week at a major public event using a walking stick, but royal officials said it was not linked to any specific health condition.
But news that she stayed overnight in a hospital will inevitably raise fears for her health, given her advanced age, and questions about whether she should slow down.
Next year she is due to celebrate her Platinum Jubilee to mark 70 years on the throne.