Gordon Brown's youngest son has been diagnosed with cystic fibrosis, the Treasury confirmed last night.
The chancellor and his wife, Sarah, were told about James Fraser's condition soon after his birth in July, although it is thought to have been confirmed only recently.
In a statement, the Treasury said the baby was "fit, healthy and making all the progress that you would expect any little boy to make".
"Thousands of other parents are in the same position. They are confident that the advice and treatments available, including proper exercise and, later, sporting activity will keep him fit and healthy," a spokesman said.
"The NHS is doing a great job, and Gordon and Sarah are very optimistic that the advances being made in medicine will help him and many others, and they hope to be able to play their part in doing what they can to help others."
There are currently about 7,500 people with cystic fibrosis in the UK. It is an inherited disorder that causes the production of a thick, sticky mucus which affects the lungs and digestive system.
According to the Cystic Fibrosis Trust, the current life expectancy for someone with the disease is about 31 years. This is an advance on 40 years ago, when diagnosed children could expect to live only about five years.
In Scotland, where Fraser was born, babies are routinely screened for cystic fibrosis. A similar system will be introduced in England from next year.
Yvette Cooper, the housing minister and wife of Mr Brown's closest political ally, Ed Balls, said last night: "Fraser is a lovely, bouncy, healthy little boy. He is doing really well.
"This is the kind of thing that no parent wants to hear. It was the same for Gordon and Sarah in the summer. But they are very optimistic, medical advances happen all the time and they are staying very positive."
She added: "They are just carrying on as normal, there are all sorts of things you can do to keep children strong and healthy as they grow, and Gordon and Sarah are doing all of that."
In an interview in September, Mr Brown spoke of his joy at the birth of Fraser, who is the couple's second son. They also had a baby daughter, Jennifer Jane, but she died aged just ten days in 2002.
Conservative leader David Cameron, whose four-year-old son, Ivan, has cerebral palsy and requires round-the-clock care, said: "Sam and I are thinking of Gordon and Sarah and their family at this time and we send them our best wishes for the future."
The Treasury released news of Fraser's condition after a tabloid newspaper inquired about a number of hospital visits.