The UK has secured an extra 60 million Pfizer vaccine doses with a view to rolling them out as part of a booster campaign in the autumn, Health Secretary Matt Hancock has said.
Officials have been planning for the possibility of having to boost COVID-19 vaccine protection for the population before the winter.
So far, almost 34 million people have already received a first shot.
Mr Hancock, who booked his own first jab earlier, said the purchase of the Pfizer/BioNTech jabs would "keep us safe and free", before he later added that the UK was on track to ease the lockdown first as expected on 17 May.
He said: "We have been working on a programme of booster shots... for over a year now, and we back some of the only clinical trials in the world looking specifically at booster shots.
"That is all about protecting the progress that we've made. We have a clear route out of this crisis, but this is no time for complacency."
He added later: "As of today, we are on track for step three (of the roadmap) on 17 May. And that is good news. We're almost exactly where the modellers predicted that we would be at this point."
There have been concerns for some time that booster jabs might be needed to protect against new variants and to sustain immunity.
Just over a week ago Mr Hancock told the Commons the government had procured enough vaccine doses to begin booster jabs later this year - adding that new vaccines were being designed to target "variants of concern" - but he did not provide any details about which vaccines would be used.
It brings the number of Pfizer vaccine doses the government has procured to 100 million - the same as the number of AstraZeneca doses ordered.
During the Downing Street news conference at which Mr Hancock made the announcement, the deputy chief medical officer Jonathan Van-Tam said COVID cases were "nearing the bottom" and, while there would be third wave, it is expected to be "much less significant".
Professor Van-Tam also said a study was under way to look at whether it was necessary to use the same vaccine someone had for their first two jabs for their booster.
Evidence has emerged today that all the current vaccines offer levels of protection that look unlikely to wane quickly.
But the explosion of cases in a country like India, many of which are said to be linked to a new variant of the virus, illustrates the need to keep protection levels at the highest they can be.
India's surge is said to be partly down to a variant called B.1.617, which features a double mutation. It is still being studied whether the variant reduces the protection offered by existing vaccines.
Pfizer and the United States have both highlighted that annual shots might be needed in the future, and Prime Minister Boris Johnson has likened COVID boosters to the annual flu shots given to the elderly and vulnerable people.
Mary Ramsay, head of immunisation at Public Health England, revealed earlier that the COVID-19 shots being rolled out are much more effective than thought possible a year ago, and have more protection than most flu vaccines.
But she told a committee of MPs that plans for booster shots will likely be driven by emerging new variants.
While there have been 148 million confirmed COVID cases around the world, there are still potentially billions of people who are yet to have been infected and consequently, large parts of the world from where new variants could emerge.
AstraZeneca has also said it aims for a new version of its vaccine to tackle variants by the autumn. The government has said the extra Pfizer doses will be used alongside other vaccines, but it is yet to confirm which ones.
Dr Ramsay said it was likely that elderly and vulnerable people were the most likely to need regular boosters, but they might not be required as often as flu shots.
Britain has recorded more than 127,000 deaths, the fifth highest death toll globally.
It comes as the latest data on vaccine effectiveness from Public Health England suggests that both the Pfizer/BioNTech and Oxford/AstraZeneca vaccines cut transmission by half.
Prof Van-Tam said, because of the level of protection they have been found to offer, it would be "incredibly safe" for two fully vaccinated people to meet up.
He suggested the reason why they are currently prevented from doing so, despite US medical authorities allowing it, was because younger people have not had a vaccine, but Mr Hancock said: "The decision we've taken... is to move together, that's what the roadmap does."
Health Secretary Matt Hancock told the briefing a decision had been made to move as a population towards greater freedoms - as set down in the Government's road map.
Also today, a new paper from the Tony Blair Institute for Global Change (TBI) said that a third wave of coronavirus across the UK is not inevitable, if the vaccine rollout continues beyond all adults in July.
ANALYSIS: Can you mix vaccines, and what will the best combinations be?
By health correspondent Ashish Joshi
When Matt Hancock announced the autumn booster programme an immediate and obvious question sprung up: can the booster jab be different to the original vaccine?
This question is prompted by the government saying it has ordered 60 million Pfizer vaccines for the campaign. So what happens to all the people who have had the Oxford vaccine?
The short answer is 'probably'.
But the longer and more correct explanation is 'probably, but we're still investigating and don't know what the best combination should be'.
According to Dr Peter English, consultant in communicable disease control, it happens with other vaccines.
He says many jabs work better if a different vaccine is used for boosting and uses the hepatitis B vaccine as an example, saying some patients respond poorly to vaccination but better when a different booster is given.
This is called heterologous boosting.
But right now we do not have the hard evidence for heterologous boosting with the current crop of UK vaccines.
Professor Jonathan Van Tam said a study into this will be launched in June.
By then many more of us will have had the vaccine and volunteers will be invited and randomised to different kinds of booster. This will then show which combination gives you the best boost.