Men should be screened for the "Celtic gene", scientists urge, as a landmark study finds it increases liver cancer risk tenfold.
Haemochromatosis, the Western world's most common genetic disorder, causes sufferers to absorb too much iron from their diet. Its accumulation around the body can cause organ damage.
More than seven per cent of men with the disorder will develop liver cancer by the age of 75, compared with 0.6 per cent of the general population, according to projections by researchers at the University of Exeter.
The faulty genes are particularly prevalent in Celtic bloodlines and some 175,000 men and boys of European ancestry in the UK are affected. Symptoms may include tiredness, muscle weakness and joint pain.
The findings have prompted renewed calls for routine early testing to be carried out for the genetic disorder. The UK National Screening Committee are currently considering whether this should be done.
Professor David Melzer, from the University of Exeter, who led the research, said: "Tragically, men with the haemochromatosis faulty genes have been dying of liver cancer for many years, but this was thought to be rare.
"We were shocked to find that more than seven per cent of men with two faulty genes are likely to develop liver cancer by age 75, particularly considering that the UK has the second highest rate of these faulty genes in the world.
"Fortunately, most of these cancers could be prevented with early treatment. Blood donations made during routine treatment of haemochromatosis can be used for other patients, so early diagnosis would actually be a win-win for the NHS."
Dr Janice Atkins, also from Exeter, added: "Unfortunately, haemochromatosis is often diagnosed too late. Earlier diagnosis could prevent so much unnecessary disease."
Using data from the UK Biobank, the researchers analysed 1,294 men with two copies of the faulty gene (one from each parent). Some sufferers only have one copy from one parent.
They found that 21 developed liver cancer and 14 died from the disease.
Ten men were not diagnosed as having haemochromatosis until they found out they had liver cancer - highlighting the need for early diagnosis.
Liver damage can lead to cirrhosis of the liver, and then potentially liver cancer.
However, once diagnosed, sufferers can be treated by having the iron levels in their blood reduced with a straightforward procedure, similar to blood donation, several times a year.
Mark Williams, 54, was diagnosed with haemochromatosis after seeking a second opinion on a sore shoulder. The doctor carried out a blood test and noticed his iron levels were high.
The father of four, from Westcliff-on-Sea, Essex, lost his father to liver cancer caused by the genetic disorder.
"My dad went his entire life with this terrible condition, and he was completely unaware," he said.
"It was devastating and such a shock. If I hadn't seen a doctor who spotted the signs, I could be looking at developing cancer in my 70s too."
Patients with haemochromatosis but no liver disease are generally advised to keep below the government’s guidance on safe drinking (no more than 14 units a week on a regular basis).
Consuming too many iron-rich foods can also have a negative impact.
Mike Smith, 66, from Oldham, Greater Manchester, became incredibly ill even by simply standing on a sea creature that absorbed large amounts of iron while he was on holiday in Tenerife.
Mr Smith, who was diagnosed with haemochromatosis in 1998, was left needing hospital treatment, antibiotics and blood tests for months afterwards.
Women are partially protected from haemochromatosis as they lose iron through menstruation and childbirth, and they were found to have no increased risk of liver cancer in the study.