A new genetics project could help "unlock a series of secrets about devastating diseases", the NHS says.
Under the scheme, 11 Genomics Medicine Centres are being set up in English hospitals to gather DNA samples to help devise targeted treatments for a wide range of diseases.
It is focusing on cancer and rare genetic diseases.
The aim is to sequence 100,000 genomes within three years in order to develop new tests and drugs.
Doctors will offer suitable patients the opportunity to take part in the scheme.
They will have to agree to have their genetic code and medical records - stripped of anything that could identify them - made available to drugs companies and researchers.
Up to 25,000 cancer patients will have the genetic code of their healthy tissue compared to the genetic code of their tumour.
A giant game of spot-the-difference will then take place to identify the precise mutations in DNA that are causing a patient's tumour.
This would allow targeted medicines to be developed.
Previous genetics research has shown how different cancers can be - for example that breast cancer is not one disease but at least 10 - each with a different cause and life expectancy and each needing a different treatment.
And the development of targeted drugs such as Herceptin - given only if a patient's breast tumour has a certain mutation - has been possible because of genetics research.
Meanwhile, 15,000 patients with rare diseases will have their genome compared with those of their parents and grandparents.
Thousands of genetic diseases - which are individually rare but combined affect large numbers of people - could be identified by finding mistakes in the three billion pairs of letters that make up our genetic code.
The resulting knowledge could give patients an explanation for a disease that has plagued their entire life.
Prof Graeme Black, who will lead the project in Manchester, told the BBC: "It's possible to sequence an individual's entire genetic make-up, their genome, in merely a few days where five years ago that was completely unimaginable.
"Therefore it's possible for conditions where there's a possibility that it's genetic, that we can identify genetic causes much quicker than had been imagined previously."
Edward Sherley-Price has first-hand experience of just how little is known about some conditions.
His 11-year-old daughter Alysia has regular seizures.
She was diagnosed with a developmental delay before her second birthday, but the cause was unknown.
It took until 2013 for genetic testing to discover what was wrong - a rare genetic mutation in a gene known as STXBP1.
The family say getting a diagnosis has given them a renewed sense of optimism.
The 100,000 Genomes Project could help people like Alysia by identifying a specific mutation, which lead to taking part in future drug trials.
The NHS said it was aiming to be the most scientifically advanced healthcare system in the world.
The 11 Genomics Medicines Centres will open across England in February at:
- Cambridge University Hospitals NHS Foundation Trust
- Guy's and St Thomas' NHS Foundation Trust, London
- Liverpool Women's NHS Foundation Trust
- Central Manchester University Hospitals NHS Foundation Trust
- Great Ormond Street Hospital NHS Foundation Trust, London
- Newcastle upon Tyne Hospitals NHS Foundation Trust
- Oxford University Hospitals NHS Trust
- Royal Devon and Exeter NHS Foundation Trust
- University Hospital Southampton NHS Foundation Trust
- Imperial College Healthcare NHS Trust, London
- University Hospitals Birmingham NHS Foundation Trust
The 11 centres are just the first wave of the project, which will eventually cover the whole of England.
Scotland, Wales and Northern Ireland are not taking part.
The pilots have been under way this year and 3,000 genomes will have been sequenced by January.
All the data produced in the 100,000 Genomes project will be stripped of anything that could identify the patient and then be made available to drugs companies and researchers to help them create precision drugs for future generations.
The project's leaders say it will be the partnership between the health service, industry and academics that will deliver a new era of genetics-based medicine.
NHS England medical director Prof Bruce Keogh said the impact of genomic medicine will be on the same scale as other British successes including the smallpox vaccine and IVF.
He said: "Our NHS is better equipped for the emerging science that will determine the future practice of medicine than any other Western healthcare system.
"[It] puts us in a position to unlock a series of secrets about devastating diseases, that have remained hidden for centuries, for the whole of human kind."
Life sciences minister George Freeman said: "We want to make the UK the best place in the world to design and discover 21st century medicines."
Angela Douglas, chairwoman of the British Society of Genetic Medicine, said: "The challenge of the project will be to embed its outcomes into routine health practice.
"The genetics community looks forward to working with researchers, scientists, associated medical specialists and Genomics England to meet that challenge."