All 16 and 17-year-olds are to be offered a first dose of a coronavirus vaccine, the Joint Committee on Vaccination and Immunisation (JCVI) has recommended, with teenagers not needing the consent of their parents to get a jab.
The UK will now follow other countries including the United States, Israel and France who have started to vaccinate older teenagers against COVID-19.
Dr June Raine, chief executive of the Medicines and Healthcare products Regulatory Agency (MHRA), said the decision to offer 16 and 17-year-olds the Pfizer/BioNTech vaccine had been taken after "rigorously reviewed" trials in children and young people.
She told a Downing Street briefing that the MHRA will "continue to scrutinise" the data as the first wave of teenagers come forward to get their jabs.
A second dose for 16 and 17-year-olds will be recommended after emerging safety data has been scrutinised, the government health advisory body said.
The first inoculations for about 1.4 million older teenagers will be offered in the next few weeks ahead of a return to classrooms for the start of the autumn term.
Professor Jonathan Van-Tam, deputy chief medical officer for England, said there is "nothing ruled out" and "nothing ruled in at this point" on the question of whether children younger than 16 will be offered a vaccine en masse.
It is "more likely rather than less likely" that the list of children eligible for vaccination will expand over time as more data becomes available, he added.
Professor Van-Tam said there was "no time to waste" in beginning inoculating 16 and 17-year-olds and stressed he wanted jabs to begin "as fast as is practically possible".
He said it was "well achievable" for the first jabs to be administered within a term, with "plentiful" supplies of vaccine on hand to be administered.
In Northern Ireland, teenagers will be able to turn up at vaccination centres on Friday and get their first jabs without an appointment.
Scotland's First Minister Nicola Sturgeon said 16 and 17-year-olds there will be offered jabs "ASAP", while Welsh health minister Eluned Morgan said the government there would work with the NHS to offer vaccinations to them.
Children will not need the consent of their parents to get a jab.
"In the UK a person who is 16 years and above is deemed able to consent for themselves, and if they are competent and able to consent for themselves then that consent holds," Professor Wei Shen Lim, COVID-19 chair for the JCVI, told the Downing Street briefing.
Health Secretary Sajid Javid said he had accepted the JCVI's recommendations and asked the NHS to prepare to vaccinate those eligible "as soon as possible".
Boris Johnson said families should listen to the advice of the experts when it comes to COVID vaccination for children.
"I would just urge all families thinking about this across the country to listen to the JCVI," the prime minister said.
"They are extremely expert there, they're amongst the best if not the best in the world, they know what's safe and I think we should listen to them and take our lead from them."
Professor Lim said: "While COVID-19 is typically mild or asymptomatic in most young people, it can be very unpleasant for some and for this particular age group, we expect one dose of the vaccine to provide good protection against severe illness and hospitalisation."
Younger children aged 12 to 15 will not be advised to get vaccinated in this phase but that could change later, with government scientists continuing to analyse data and evaluate any risks.
In July, the JCVI said children that age who have an underlying health condition that puts them at risk of severe COVID will be offered a vaccination.
It was also recommended that 12 to 15-year-olds who live with or are close family contacts with someone who is deemed at risk should be offered a vaccination, while healthy children who are less than three months away from their 18th birthday are also able to get a jab.
There has been significant debate over whether younger individuals should be offered the jab.
Some scientists say it would prevent further disruption to schooling in the next academic year, but other individuals have suggested that - as children are at a lower risk of serious illness from the virus - it would not be beneficial.
According to the JCVI, young people will have 80% protection against hospital admission once they have received their first dose.
It said that information which caused it to reconsider its stance on vaccinating 16 and 17-year-olds included the recent increase in infections, more data on the safety of vaccines and the excellent progress of the adult inoculation programme.
The JCVI said it weighed up a number of factors, but considered the most important to be the risk/benefit of vaccination to the individual.
Experts considered reports of heart inflammation among some younger adults who had been given a jab, but officials said this was deemed to be "extremely rare", affecting around one in 100,000 people vaccinated.
The effects were described as "mild", with a short recovery period.
The experiences of children who have received the vaccine in clinical trials and real world data suggest that some experience short-lived side effects, including fever, sore arm, headache and tiredness.