The Queen is spending a second day in hospital where she is being assessed for symptoms of gastroenteritis.
She was taken to London’s King Edward VII Hospital from Windsor Castle, where she had been resting, on Sunday. It is her first hospital stay in 10 years.
Buckingham Palace said the Queen, 86, had been admitted as a precaution and was otherwise in “good health”.
All official engagements for this week, including the Queen’s trip to Rome, will be either cancelled or postponed.
News of her illness first emerged on Friday night and she cancelled a trip to Swansea on Saturday to mark St David’s Day in a military ceremony.
However, earlier on Sunday she had carried out a private medal presentation at Windsor Castle.
The Queen has spent the night in reasonably unfamiliar surroundings – a private hospital she was last a patient at 10 years ago.
Then she was having a non-cancerous growth removed from her face and an operation on her knee.
Now, as a precaution, says the palace, she is being assessed for what it’s calling “symptoms of gastroenteritis”.
British monarchs, unlike present day American presidents, aren’t obliged to publish detailed medical bulletins.
So, we know the 86-year-old monarch is said to be in “good spirits” and isn’t bedridden, but the nature of her treatment hasn’t been made public.
It could include tests to establish if the symptoms she’s been suffering from have been caused by an infection or an underlying problem.
As things stand, the Queen is expected to leave hospital soon and at some stage she will resume the role to which she has committed much of her adult life.
There was no sense of alarm from palace officials about the events, our correspondent added.
The Queen last received treatment 10 years ago, again at the King Edward VII hospital. for a minor knee operation. At the same time, surgeons removed minor, non-cancerous lesions from her face.
Gastroenteritis causes inflammation of the stomach lining and intestines.
The infection can be transmitted through contact with an infected person or contaminated food and drink. Symptoms can include vomiting, fever and stomach ache.
The Queen’s treatment, which has not been disclosed, could include rehydration and tests to establish if the illness has been caused by an infection or an underlying problem.
The Queen had been due to spend two days in Rome with the Duke of Edinburgh next weekend, at the invitation of Italy’s President Giorgio Napolitano.
It is not clear whether the visit will be re-scheduled.
A reception at Buckingham Palace on Tuesday for MPs and MEPs will go ahead with other members of the royal family present.