People aged 30 and 31 can now book to have their first and second doses of the COVID-19 vaccination.
The NHS is urging people to take up the offer of the jab, and to book appointments for both the required doses at the same time (between eight and 12 weeks).
Around one million people aged 30 and 31 will get a text message in the coming days asking them to come forward, as Britain attempts to ramp up vaccinations.
Pressure has been increased by the threat of the B.1.617.2 variant.
Health officials sped up the targets timetable to ensure those at the highest risk are protected from the virus first identified in India.
Those considered clinically vulnerable are having their second dose brought forward to eight weeks due to concerns about the new strain.
Concentrated hotspots have been identified as Bedford, Blackburn with Darwen, Bolton, Burnley, Kirklees, Leicester, Hounslow and North Tyneside.
Guidance says people in these areas should try to avoid meeting indoors, as a precaution.
The government plans to offer the first doses to all adults by the end of July.
Health Secretary Matt Hancock said: “Our vaccination programme is moving at such a phenomenal pace and I am delighted that less than six months after Margaret Keenan received the first authorised jab in the world, we are now able to open the offer to everyone in their thirties and over.
“The vaccine is our way out of this pandemic and recent data has shown the life-saving protection [that] a second dose of the jab can give, especially against the new variant.
“I urge everyone to come forward when you get the offer and play a part in getting us back to normality.”
Text invitations appear as an alert from ‘NHSvaccine’, including a web link to the NHS website to reserve an appointment. More appointments are being added to the National Booking Service every day.
People who cannot go online can call the service on 119 instead to book their jab.
People 39 and under who are eligible and pregnant women will be offered the Pfizer or Moderna vaccine in line with recently updated JCVI guidance.