Deputy Prime Minister John Prescott is suffering from pneumonia, his office has confirmed.
John Prescott was taken ill on Saturday
The 69-year-old has been moved to a high-dependency unit, but a spokesman stressed that this was not because his condition had deteriorated.
He has been in hospital since Saturday, when he was taken ill on a train from Hull to London.
His friends have told the BBC's Nick Robinson that Mr Prescott is conscious and there is no sense of alarm.
Our political editor said the friends had told him that the deputy prime minister was sitting up, and had managed to dictate a statement about his illness.
It is thought he was moved to the high dependency unit - for patients needing more observation and treatment than in a general ward - owing to his age and the fact that he suffers from diabetes.
Mr Prescott's wife, Pauline, has been at his bedside on Tuesday.
His official spokesman said: "He has been moved to a high-dependency unit, not because of a deterioration in his condition but so that his condition can be monitored more closely."
Cabinet minister Peter Hain, who supported Mr Prescott's campaign to be Labour's deputy leader in 1994, said everyone in the party was wishing him a speedy recovery.
"I have sent my best wishes to John and his family," said Mr Hain, who is one of the candidates to succeed Mr Prescott in his party role.
In May, Mr Prescott announced he would step down as Labour deputy leader and deputy prime minister on 27 June - the same time as Prime Minister Tony Blair.
During his 10 years as Mr Blair's deputy, Mr Prescott has been generally in good health.
However, in 2002 it was announced he had suffered from type two diabetes for a number of years, managing the condition with medication.
And he was taken to hospital on Christmas Day 2006, when he was treated for kidney stones.
Pneumonia is an inflammation of the lungs, usually caused by an infection.
Many different bacteria, viruses and even fungi can bring on the illness - although Mr Prescott's official spokesman said he did not know what had caused the pneumonia in his case.
Symptoms include difficulty in breathing, chest pain, coughing and fever. It is frequently resolved within weeks.