Up to 1,000 seriously un-well children will benefit from new fast-tracked genetic testing
The NHS have described the new scheme as "revolutionary"
A new scheme in England will offer fast-tracked genetic tests to any seriously unwell children whose doctors cannot diagnose their illness.
The NHS said around 1,000 babies and children in intensive care will benefit each year from the "revolutionary" scheme.
NHS England said its national genetic testing service is a "world first" and could "save the lives of thousands" of severely ill youngsters.
Children to receive treatment within a week
When doctors suspected genetic illness for a child's symptoms previously, the youngster may have needed a battery of tests with results sometimes taking weeks.
Now children will only need to provide simple blood tests, which will be sent off for genetic testing at a Devon laboratory. The results will be returned within seven days so treatment can start within a week.
The new scheme, announced on Wednesday by NHS England boss Amanda Pritchard, was trialled in March in the South West. It is now being rolled out across the whole of the country.
During the trial, a newborn's life was saved after his blood was sent off for rapid testing.
In March, Reuben Tanaka, from Cheltenham, Gloucester, fell badly ill at five days old after vomiting and refusing to feed.
The baby, now seven months, was admitted to Bristol Royal Hospital for Children's intensive care unit, with potentially lethal levels of ammonia in his blood. Doctors suspected urea cycle disorder, but tests were inconclusive.
Thanks to the trial of the rapid whole genome sequencing service, Reuben did not need an invasive and potentially dangerous liver biopsy - with blood tested instead.
Genetic changes in the CSP1 gene were found, meaning the youngster's body could not break down nitrogen which, in turn, caused the toxic ammonia levels. The correct medication was quickly given to Reuben, saving his life.
"When the doctor came out and told us that they were doing everything they could keep him alive, what we were facing really dawned on us," said his 39-year-old father Atsushi.
"We saw him connected to 20, 30 tubes and it was really tough to see."
Reuben's mother Eleanor, 38, added: "All the care Reuben received would not have happened as quickly and his early diagnosis meant we knew what to expect.
"We're so grateful for everything the NHS has done to enable this to happen and the incredible genomic testing we have had."
New era in genomic medicine
After two and a half months in hospital, Reuben was discharged and he is now doing well while waiting for a liver transplant, which will cure his condition.
Ms Pritchard said the new service is the start of a "new era in genomic medicine".
"This global first is an incredible moment for the NHS and will be revolutionary in helping us to rapidly diagnose the illnesses of thousands of seriously ill children and babies - saving countless lives in the years to come," she will tell delegates at the Genomics Healthcare Summit in London.
"I have seen how these simple blood tests can change the lives of babies and their families and being able to expand this further is wonderful for children and their families.
"When a child comes to intensive care, timing is everything. So finding the right diagnosis and treatment as quickly as possible is absolutely vital, and I am delighted that the pioneering work of the NHS's Genomic Medicine Service is transforming the way we diagnose and treat patients in England.
"The NHS is recognised worldwide as a world leader in genomics and this new service proves just that. It also builds on our Long-Term Plan commitment to deliver the most medically advanced services possible for all our patients - boosting the life chances of thousands across the country."
5-year plan to expand current genomic testing
Ms Pritchard is also expected to announce a five-year plan to enhance genomic medicine in the health service, which will include expanding current genomic testing in the NHS and a 'Genomic Training Academy', which will help teach current NHS staff about the importance of genomic medicine.
Dr Emma Baple, who runs the National Rapid Whole Genome Sequencing Service, said: "The rapid whole genome testing service will transform how rare genetic conditions are diagnosed.
"We know that with prompt and accurate diagnosis, conditions could be cured or better managed with the right clinical care, which would be life-altering - and potentially life-saving - for so many seriously unwell babies and children.
"This new service, which has been over five years in the making, is a huge step-change in what we can do for these children and their families, and it is an absolute privilege for us to play a part in helping to give children up and down the country the best care possible."