The Queen was back at her desk carrying out "light" work after spending the night in hospital.
The 95-year-old monarch underwent “preliminary investigations” at the private King Edward VII's Hospital in London, according to Buckingham Palace.
It came just hours after cancelling a planned trip to Northern Ireland this week, reports the Mirror.
In a statement on Thursday night, a Palace spokesperson said: “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.”
The Queen is now recovering at Windsor Castle, where she has spent most of the last 18 months amid the Covid pandemic
A royal source has said she was undertaking some light duties and reading her official government red boxes on Thursday.
It is believed that the overnight hospital stay was for practical reasons, with the visit not announced by Buckingham Palace to protect her medical privacy.
The source told the Daily Mail: "Quite understandably, a cautious approach is being taken by the medical team.
"Her Majesty was back at her desk this (Thursday) afternoon and undertaking light duties."
Palace officials have moved to quash concerns over the Queen’s health.
They insist the monarch was told to cancel the trip for Wednesday and Thursday after doctors ordered a rest period.
In a statement, a spokesperson said: “The Queen has reluctantly accepted medical advice to rest for the next few days.
“Her Majesty is in good spirits and is disappointed that she will no longer be able to visit Northern Ireland.
“The Queen sends her warmest good wishes to the people of Northern Ireland, and looks forward to visiting in the future.”
The Queen has travelled nearly 1,000 miles this month, including returning from the royal estate in Aberdeenshire, where she spent the summer.
It is the first time the Queen has stayed in hospital since 2013, when she suffered symptoms of gastroenteritis.
She has also had a successful surgery to treat an eye cataract in 2018, and a knee operation in 2003.
Earlier this month, she was seen using a walking stick for support in public for the first time, apart from after her knee operation.
Courtiers believe the cancellation of the Queen's trip serves as a reminder that the head of state cannot do what was expected of her 10 or 20 years ago.
Her next major public engagement is at the end of the month when she is due to lead a royal delegation and welcome world leaders at the opening of the United Nations climate change summit in Glasgow.
