University Hospitals of Leicester NHS Trust has apologised after a blunder with test results delayed treatment for three brothers diagnosed with the same terminal illness.
Six-year-old Callum Smith, and siblings Connor, nine, and Jack, five, all have adrenoleukodystrophy, a rare genetic condition (ALD) which could claim their lives in just a few years if no cure can be found.
However, because it emerged doctors failed to spot that a cousin also has the brain disease, vital treatment for the boys is two years late.
Callum has the worst form of the illness and is no longer strong enough to undergo a bone marrow transplant, which can slow the progress of ALD.
The boys' cousin, Neil Caven, 33, has adrenomyeloneuropathy (AMN), an adult form of the condition.
He was blood-tested in December 2003 at Leicester Royal Infirmary but the sample took so long to reach the laboratory it was not fit for checking.
Further tests in May last year also left him undiagnosed. It meant the disease went undiscovered for two years and four months.
Delayed treatment caused by late diagnosis
Mr Caven only found he had the illness this year after his young cousins were diagnosed with ALD.
A University Hospitals of Leicester NHS Trust spokesman said: "Mr Caven's medical notes show that he did have a blood test in 2003 which could have led to an earlier diagnosis of the disease had the results been reported back.
"The fact that the results were not reported back is our fault and we are sorry. We are taking steps now to improve the way that blood samples are sent for testing."
"We are still looking after the boys and we, like their family, continue to hope that there is a medical breakthrough in the treatment of ALD."
The boys' parents, who live in Eyres Monsell, Leicester, have been left stunned by news of the blunder.
Their mother, Nicola Smith, 26, told the Sun newspaper: "I didn't believe things could get any worse until I heard this dreadful news. There's no doubt it would have made a difference if we had known of Neil's condition and the boys had been tested earlier."