Professor Dame Sue Hill, chief scientific officer for England and senior officer for genomics at NHS England, said the rollout marks "an important moment" for how genomics can improve the safety of cancer treatments.

She said: "As our understanding of the role our DNA plays in disease grows, we will be able to use this approach to help develop personalised treatments for other conditions and embed genomics into routine care."

Dr Simon Vincent, research director at Breast Cancer Now, said the test is a "welcome step" towards ensuring the "kindest" modes of treatment are used.

The test works by detecting variants within a specific gene, known as DPYD, which are likely to mean a patient has a lower level of the enzyme needed to break down cancer drugs in the body, leaving them exposed to higher levels of toxicity.

John McGuire, 71, from London, who is undergoing chemotherapy for colorectal cancer at Guy's Hospital, was put on a lower dose after being found to have the form of the gene.

Mr McGuire, who is halfway through treatment which aims to cure his cancer, said: "I'm delighted with the treatment I have received from the team at Guy's Hospital and have had little to no side-effects from my treatment... I think I am going to be really happy with the outcome."

It marks the latest in a series of innovations and genomic discoveries adopted by the NHS to deliver personalised cancer care.

Cancer survival rates are already at a record high, but the NHS Long Term Plan aims to catch three-quarters of tumours at an early stage, when they are easier to treat.

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Cancer patients to be offered test that tailors treatments to their genetics to avoid toxic side-effects - Telegraph.co.uk