The milestone comes a day after the Government announced that three-quarters of adults had received their first dose.
A total of 26,422,303 second doses have now been delivered since the vaccination rollout began almost six months ago.
This is the equivalent of 50.2% of all people aged 18 and over.
In England, 22,442,383 second doses have been given – the equivalent of 50.7% of the adult population.
England is slightly ahead of the other three nations of the UK, with Scotland on 48.2% (2,137,618 second doses), Northern Ireland on 47.1% (684,398 second doses) and Wales on 45.9% (1,157,904 second doses).
Vaccines minister Nadhim Zahawi tweeted: “Another important milestone today. 50% of all adults in the UK have had the protection of two doses. Thank you to a brilliant team. We keep going.”
The latest figures have been published by the UK’s four health agencies.
They also show that an estimated 75.5% of UK adults have now received a first dose of vaccine.
In Wales 85.7% of adults are estimated to have had a first jab, some way ahead of England (75.0%), Scotland (74.6%) and Northern Ireland (73.9%).
The Government has said it intends to offer a first dose of a vaccine to all adults by the end of July, and both doses to everyone aged 50 and over by June 21.
It comes as expert Professor Robin Shattock warned that despite the continued success of the vaccine rollout, the public should not see themselves as living in “Fortress UK” because the rest of the world needs to get access to jabs too.
Professor Peter Openshaw, who joined him for the Royal Society of Medicine webinar on Thursday, branded it “unethical” not to share surplus vaccines with the rest of the world.
He called for the “over purchasing that we and others have done” to be addressed “so as to reduce transmission, and reduce disease amongst those who will definitely benefit a great deal (from vaccines)”.
Prof Shattock, head of mucosal infection and immunity at Imperial College London said: “We’ve done incredibly well in the UK with rollout of vaccines but we can’t think of ourselves as Fortress UK.
“We need to get those vaccines rolled out to the rest of the world.”
Additional reporting by the Press Association.