Confusions increased over the queen's health after the palace hid that she spent one night at the hospital.
Questions mounted about the health of 95-year-old Queen Elizabeth II after she had tests and spent a night in hospital, despite royal officials saying she was resting at home.
After the recent development, British Prime Minister Boris Johnson has passed his best wishes to Queen Elizabeth, his spokesman said on Friday.
On Wednesday morning, Buckingham Palace had said that the queen pulled out of a planned engagement in Northern Ireland and had been advised to rest on medical advice.
But royal officials were forced to confirm late Thursday that she had in fact had "some preliminary investigations" in hospital on Wednesday, and stayed overnight after The Sun newspaper broke the news.
Britain's Press Association news agency said the hospital visit and stay were not initially disclosed because it was expected to be short and to protect her privacy.
Later on, it was said that she returned from King Edward VII's Hospital in central London to her Windsor Castle home west of London and was said to be "in good spirits".
"God save the queen"
The disclosure, however, inevitably raised fears for her health and the toll of her public duties given her advanced age.
Veteran BBC royal correspondent Nicholas Witchell said royal officials "have not been giving us a complete, reasonable picture of what has been occurring".
"Rumours and misinformation proliferate (and) thrive when there is an absence of good, proportionate, trustworthy information.
"We must hope that we can rely on what the palace is now telling us," he added, calling assurances that the queen was in good spirits "a handy phrase that the palace dusts off at moments such as this".
Richard Palmer, the royal correspondent at the Daily Express, also called the phrase a "palace cliché".
"Royal sources had been keen to encourage the impression that she had just overdone it but may struggle to convince the public now," he noted.
Previously, the queen's late husband, Prince Philip, died in April just a few weeks before his 100th birthday, months after spending four weeks in hospital receiving treatment for a pre-existing heart condition.