The Queen was back at her desk and doing "light" work after being forced to spend the night in hospital, it is reported.
The 95-year-old monarch underwent “preliminary investigations” at the private King Edward VII's Hospital in London just hours after cancelling a planned trip to Northern Ireland this week, according to Buckingham Palace.
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 recovering at Windsor Castle where she has spent most of the last 18 months due to the Covid pandemic, and 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 and that the visit had not been announced by Buckingham Palace to protect her medical privacy.
A source said the Queen carried on with her work after returning home.
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 quickly to dispel any alarm for the Queen’s health, saying the monarch had been told to cancel the trip for Wednesday and Thursday after doctors insisted on a period of rest.
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.”
Her admission is understood not to have been related to coronavirus, and followed a busy period of official engagements.
She has travelled nearly 1,000 miles this month, including returning from the royal estate in Aberdeenshire, where she spent the summer.
An official record of her diary showed at least 16 formal events in October as well as the planned two-day trip to mark the 100th anniversary of the partition of the island.
She had spent Tuesday evening hosting a drinks reception for a Global Investment Summit at Windsor Castle alongside Boris Johnson.
At the reception, she appeared to be in good health. 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.
Courtiers believe the cancellation of the Queen's trip will serve as a reminder that the head of state - at the age of 95 - cannot do what was expected of her 10 or 20 years ago.
Earlier this month, she was seen using a walking stick for support in public for the first time, apart from after her knee operation.
It is understood that courtiers are keeping a close eye on the Queen’s diary moving forward so that she has the correct amount of time to recuperate from her schedule.
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.
Aides have said they are still confident that she will be well enough for COP26 and there were no plans to cancel her attendance.
