Update: Queen Elizabeth II was released Monday morning, according to CNN.
Queen Elizabeth II was spending Monday in hospital suffering from the symptoms of gastroenteritis, having been admitted for the first time in 10 years.
The British monarch was not expected to leave the private King Edward VII's Hospital in central London before Tuesday.
A planned visit to Rome this week -- which would have been her first trip abroad since October 2011 -- has been abandoned and there has been no announcement on whether it will be rescheduled.
"There is no update on her condition," a Buckingham Palace spokeswoman told AFP.
"The queen is still in hospital."
The palace said Sunday that the sovereign was in good spirits and otherwise in good health.
Britain's head of state has enjoyed robust health and rarely misses engagements.
Illustrating her determined spirit, the queen reportedly presented a long-service medal to a staff member before being driven to hospital on Sunday.
The sovereign, accompanied by her 91-year-old husband Prince Philip, was to visit Rome on Wednesday and Thursday, but all her engagements for this week have now been called off.
The royals were to receive a ceremonial welcome, meet Italian President Giorgio Napolitano and attend a private lunch, before visiting the Pantheon monument.
There was widespread support for the queen in the British press on Monday.
The Sun newspaper said that while hundreds of thousands of Britons take the day off sick with stomach complaints every day, "the one person who you can be sure isn't swinging the lead is the Queen.
"Even at 86, Her Majesty rarely lets up on her still-busy schedule.
"Such is her total devotion to duty that she doesn't often let a little thing like ill health get in the way of her work."
The Daily Mail newspaper said the queen was "as strong as an ox -- but do we ask too much of her?"
"She shows no sign of slowing up, and the allowances she makes for age are purely marginal."
While just a few officers are stationed outside the entrance to the hospital, there is certainly a big police presence.
Anthony Wallyn, Britain's tallest policeman at seven feet, two inches (2.18 metres), has been among the officers standing guard at the entrance.
The 26-year-old admitted it would produce an unusual image if Queen Elizabeth -- who is five feet, four inches (1.63 metres) tall -- walked past him when she left hospital.
"It would be a nice photo," he joked.