More than a million people most at risk of needing hospital treatment if they catch coronavirus are being asked to stay at home for at least 12 weeks.
This "shielding" is to protect lives, with those affected being written to over the next couple of days.
Why do it?
Cases of coronavirus are increasing rapidly in the UK, which suggests it is circulating in the community and spreading from person to person.
While everyone is being advised to keep their distance from other people to help stop the spread, some people with underlying health conditions need to take even more precautions to protect themselves.
Who should do it?
Those most at risk, who include:
Solid organ transplant recipients
Some people with cancer who are undergoing treatments such as chemotherapy and radiotherapy
People on immunosuppression drugs
Women who are pregnant and have heart disease
People with severe respiratory conditions - cystic fibrosis, severe asthma and COPD
Some people with rare diseases such as severe combined immunodeficiency
Anyone in this highest risk category who has not received a letter from the NHS by Sunday 29 March 2020 or been contacted by their doctor should get in touch with their GP or hospital doctor by phone or online.
This does not include all elderly people, although they are strongly advised to practice social distancing and only go out if really necessary.