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Gordon Brown and his wife Sarah have been hit by a fresh challenge after it was discovered that their baby son has cystic fibrosis.

Mr Brown, who is expected to succeed Tony Blair as Prime Minister, learnt of the condition of his four-month old son Fraser soon after his birth, following a series of blood tests.

It follows the death of their first baby in 2002. Jennifer Jane was born seven weeks premature and suffered a brain haemorrhage shortly before she died. They have a three-year-old son, John, who is fit and healthy.

A Treasury spokesman said last night that Fraser had had cystic fibrosis diagnosed. But he was healthy and “making all the progress you would expect a little boy to make”.

He added: “They were told in late July that Fraser may have cystic fibrosis. Tests since then have confirmed this.

“Thousands of other parents are in the same condition. They are confident that the advice and treatments available, including proper exercise and later sporting activity, will keep him fit and healthy.

“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.”

Cystic fibrosis is most common life-threatening inherited disease in Britain and affects more than 7,500 babies, children and young adults. The average life expectancy is 31.

The symptoms can include poor weight gain, troublesome coughs and repeated chest infections.

Fraser Brown was born on July 17 at the Royal Infirmary, in Edinburgh. He weighed a healthy 7lb 14oz (3.6kg).

The condition was picked up in routine blood tests that all babies in Scotland undergo.

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