The Queen will no longer travel to Rome this week after being admitted to hospital, Buckingham Palace has said.
The Italian presidency has confirmed the scheduled two-day visit has been cancelled due to "health reasons", Reuters reported.
Buckingham Palace has confirmed that the Queen is being assessed at the King Edward VII Hospital, London after experiencing symptoms of gastroenteritis.
This is the same hospital where the Duchess of Cambridge recently received treatment for morning sickness.
The Queen will remain in hospital for two days as she receives treatment for gastroenteritis, BBC News has reported.
Buckingham Palace has confirmed the Queen has been admitted to hospital in London after experiencing symptoms of gastroenteritis.
All official engagements for this week will be either postponed or cancelled as a precaution, the Palace added.
The Queen is in hospital suffering from gastroenteritis, Sky News has reported.
The Queen is expected to spend another day at Windsor Castle as she continues to recover from her stomach illness.
As ITV News' Lewis Vaughan Jones reports, her appearance at a St David's Day military celebration in Swansea today had to be cancelled - and a trip to Italy next week is still in doubt.
The Changing of the Windsor Castle Guard has taken place as usual today.
The Queen is spending the day resting at the castle while cancelling a visit to Swansea after a bout of gastroenteritis.
The Queen will spend the day resting after suffering symptoms of gastroenteritis, Buckingham Palace said.
She was due to attend a military celebration in Wales today during the city's St David's Day celebrations, but has been forced to cancel her planned engagements because of the sickness.
She will instead spend the weekend at Windsor, a palace spokeswoman said.
The Queen has cancelled a planned visit to Swansea tomorrow because she is "experiencing symptoms of gastroenteritis".
Instead, she will stay at Windsor Castle over the weekend.
ITV News correspondent Geraint Vincent reports:
A St David's Day ceremony the Queen was due to attend tomorrow will go ahead as planned with the Lord Lieutenant of West Glamorgan, Byron Lewis, taking her place.
A Swansea Council spokesman said:
The ceremony will continue as planned and will be carried out by the Lord Lieutenant of West Glamorgan, Mr Byron Lewis.
The Lord Lieutenant will be met upon arrival at the Guildhall by Brigadier Philip Napier, Colonel of the Regiment.
The parade will present arms for a royal salute and play the national anthem before the Lord Lieutenant is invited to present leeks to a representative body of the regiment.