First doses of the Oxford-AstraZeneca coronavirus jab are to be given as the UK accelerates its vaccine programme to tackle a surge in cases.
More than half a million doses of the vaccine are ready for use on Monday.
The health secretary described it as a "pivotal moment" in the UK's fight against the virus, as vaccines will help curb infections and ultimately allow restrictions to be lifted.
But the PM has warned tougher virus rules may be needed in the short-term.
Boris Johnson said regional restrictions in England are "probably about to get tougher" as the UK struggles to control a new, fast-spreading variant of the virus.
On Sunday more than 50,000 new confirmed Covid cases were recorded in the UK for the sixth day running, prompting Labour to call for a third national lockdown in England.
Northern Ireland and Wales currently have their own lockdowns in place, while Scottish cabinet ministers will meet later to consider further measures.
Six hospital trusts - in Oxford, London, Sussex, Lancashire and Warwickshire - will begin administering the Oxford-AstraZeneca jab on Monday, with 530,000 doses ready for use.
Most other available doses will be sent to hundreds of GP-led services and care homes across the UK later in the week, according to the Department of Health and Social Care (DHSC).
'End in sight'
Health Secretary Matt Hancock said: "This is a pivotal moment in our fight against this awful virus and I hope it provides renewed hope to everybody that the end of this pandemic is in sight."
But he urged people to continue to follow social distancing guidance and coronavirus rules to "keep cases down and protect our loved ones".
As the recent rise in Covid cases puts increased pressure on the NHS, the UK has accelerated its vaccination rollout by planning to give both doses of the vaccine 12 weeks apart, having initially planned to leave 21 days between jabs.
The UK's chief medical officers have defended the delay to second doses, saying getting more people vaccinated with the first jab "is much more preferable".
BDST: 1238 HRS, JAN 04, 2021