Second Covid-19 vaccine doses will be accelerated for the over-50s and vulnerable people, Boris Johnson has announced.
Some 10 million people could see their second doses delivered within eight weeks of their first, rather than 12 weeks.
It comes amid rising concern over the Indian variant of Covid-19, which is surging in London, Merseyside and the North West.
The Prime Minister warned the Indian coronavirus variant could "pose a serious disruption" to plans to ease restrictions and "could make it more difficult" to end them as hoped in June.
He told the Downing Street press conference: "I do not believe that we need, on the present evidence, to delay our road map and we will proceed with our plan to move to step three in England from Monday.
"But I have to level with you that this new variant could pose a serious disruption to our progress and could make it more difficult to move to step four in June."
The PM urged people to "think twice" ahead of travelling to areas with higher incidences of the Indian variant and staying with family and friends within those areas.
He said: "I would urge people just to think twice about that.
"We want people in those areas to recognise that there is extra risk, an extra threat of disruption to progress caused by this new variant and just to exercise their discretion and judgment in a way I'm sure that they have been throughout this pandemic."
Currently people aged 38 and over have been invited to book appointments for vaccinations across the UK.
Pharmacists in Sefton on Merseyside began inviting over 20s to get the jab on their own initiative this afternoon.
And earlier the leader of Bolton Council said there had been discussions with the Government about the introduction of vaccines for younger age groups and suggestions were it would be progressed as soon as possible.
Asked about surge jabs, David Greenhalgh told the BBC: "This is an issue of capacity but we have had very, very constructive talks and certainly all the soundings are is that they are looking to progress that as soon as possible.
"The vast majority of our cases are in their teens, 20s and 30s at the moment.
"If we can get vaccinations to (those aged) 16-plus, which are licensed by Pfizer, then it will make a total transformation of transmission as it moves forward.
"I think realistically that would start in targeted areas where we see the predominance (of the virus) currently."
London and the North West have seen the biggest rise in cases of the variant, with Public Health England (PHE) data showing it has been responsible for four deaths as of May 12.
The Government has been looking at ways to "flex" the rollout of vaccines in the worst-hit areas, including possibly vaccinating everyone in multi-generational households from 18-year-olds to grandparents.
More vaccine doses have been sent to Bolton, which has a particularly high rate of the Indian variant, while 800,000 PCR tests have been sent to 15 separate areas of England, including parts of London and Merseyside.
