Two million text messages are being sent out to those aged 50-54 in England with a link allowing people to book an appointment through the national booking service website.
People who cannot go online can call the service on 119.
This is the final group on the priority list which covers 99% of those at high risk of dying from Covid-19.
Everyone in the top nine priority groups should be offered a second dose by mid-July, up to 12 weeks after the first.
Nearly half of the UK adult population has had a jab - more than 24 million people.
Around 1.6 million people have also had a second dose.
Health Secretary Matt Hancock said: "I'm determined no-one should miss out on the chance to protect themselves and urge everyone who is eligible to come forward."
Click HERE or on the image above to read further details.