GORDON BROWN last night revealed that his four-month-old son James Fraser has been diagnosed with cystic fibrosis.

A spokesman for the Chancellor said he and his wife, Sarah, were told their youngest son might have the condition shortly after his birth during the summer.

The spokesman said: "While Gordon and Sarah's younger son, Fraser, has been diagnosed with cystic fibrosis, he is fit, healthy and making all the progress that you would expect any little boy to make.

"They were told in late July that Fraser may have cystic fibrosis. Tests since then have confirmed this.

"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."

Housing Minister Yvette Cooper, a close friend of the Browns' along with her husband, Economic Secretary Ed Balls, said they were a "strong and happy family".

She said: "Fraser's a lovely, bouncy healthy little boy. He's doing really well.

"This is the kind of thing that no parent obviously wants to hear, and it was the same for Gordon and Sarah in the summer.

"But they're very optimistic and doing really well, and medical advances obviously happen all the time.

The Browns were hit by tragedy in January 2002 when their first child, Jennifer Jane, died ten days after her premature birth.

But the couple were overjoyed in 2003 with the birth of their oldest son, John, and the family grew again in July with the birth of James Fraser, weighing 7lb 14oz.

Asked about the earlier tragedy, Mrs Cooper said: "I think obviously this is very different. Fraser is a lovely part of their family and they are all enjoying life and getting on with things.

"He's doing very well. This is a very different situation now."

Conservative leader David Cameron's four-year-old son, Ivan, suffers from cerebral palsy.

Mr Cameron said last night: "Sam (his wife) and I are thinking of Gordon and Sarah and their family at this time, and we send them our best wishes for the future."

Ed Owen, a former advisor to Leader of the Commons Jack Straw, and who knows Mr Brown, is a board member of the Cystic Fibrosis Trust. He has a four-year-old daughter with the condition.

Mr Owen said he had spoken to the Chancellor, and could empathise with what the couple were going through.

He said: "Children with cystic fibrosis do have to undergo a daily regime of medication and therapy, but given the extraordinary development in treatments over recent years, children can expect to live an extremely happy, active and in most ways normal life."

Mr Owen said Mr Brown had helped raise funds for the trust before Fraser was born.

"He was always very supportive of the trust, and two years ago hosted a reception at Number 11. He made a speech and has been very supportive of efforts to raise funds.

"It's something that he personally had been aware of through the charity work that he has been doing."

James Fraser, known as Fraser by his family, was born at Edinburgh Royal Infirmary on July 17.

His brother, John, now aged three, is said to have chosen the name Fraser, while James is a family name.

Mr Brown was understood to be at 11 Downing Street with his family last night.