Letters are being sent to more than 65,000 retired doctors and nurses in England and Wales asking them to return to the NHS to help tackle the coronavirus outbreak.
Senior officials say the ex-employees are needed to boost frontline services.
It comes after the government pledged to ensure that all hospitals have enough protective gear and ventilators.
In Scotland, anyone who left the medical profession during the past three years has also been asked to consider returning to the NHS.
Meanwhile, the chancellor is set to announce a wage subsidy package to protect jobs.
Many firms are warning of collapse, wiping out thousands of jobs, as life in the UK is largely put on hold.
One proposal under discussion is for the UK to follow the lead of countries such as Denmark, where the government has promised to cover 75% of salaries at private companies for three months, if they promise not to let staff go.
In the UK, 144 people with the virus have died, and 3,269 people have tested positive for Covid-19.