Flu vaccination will be offered to people aged 50-64 under the expanded NHS programme, increasing the list of eligible patients by more than 12m.
Households of the 2.2m patients on COVID-19 shielding lists and children in the first year of secondary school will also be offered the vaccination this year, the government has announced.
Government ministers have been saying for weeks that this year's flu vaccination campaign would be the biggest in history - but GPs have been in the dark until now about how the programme would change.
GP leaders warned that the massive expansion of the flu programme leaves practices needing to amend orders for vaccines made at the start of the year, and called for assurances from the government that enough doses would be available.
The RCGP said delivering the campaign while keeping vulnerable patients safe would 'present challenges' - and called for more detail on how the programme will work.
The DHSC has said that vaccinations for patients aged 50-64 will be rolled out 'once vaccination of the most at-risk groups is well underway' - with the government promising to 'work with clinicians to decide' when the rollout to this group should go ahead. Patients in this group will be contacted directly 'including information about where to go to get the vaccine', a DHSC statement said.
Confirmation of the scale of the planned vaccination campaign for this year - prompted by fears that a challenging flu season could place unsustainable pressure on the NHS if it came alongside a second wave of coronavirus - comes amid warnings that vaccination will take much longer to administer.
RCGP guidance published earlier this week warned that flu vaccinations could take more than twice as long this year because of the need to observe social distancing rules and the need for clinicians to change personal protective equipment (PPE).
In the 2019/20 flu campaign, around 15.3m vaccinations were administered to eligible groups, covering patients over 65, those in clinical at-risk groups, pregnant women, children aged two to three years old, primary school children and healthcare workers.
NHS England is predicting increased uptake among groups previously eligible for flu jabs because of health fears triggered by the pandemic - and has said it expects 'universal' uptake among healthcare staff.
RCGP chair Professor Martin Marshall said: 'This announcement of an extension to the flu programme is sensible as we prepare the NHS for a busy winter and potential second wave of COVID-19, but GPs and our teams, who will be delivering the majority of vaccinations, need more detail about the practicalities of how it will work.
'Practices plan meticulously for the flu season every year to ensure the vaccination programme runs smoothly and as many people as possible get vaccinated – they will have made their orders at the beginning of the year and will need to amend these. We also need assurance that the government can guarantee adequate supply for everyone covered under the extension.
'It is likely that COVID-19 will present challenges to delivering the flu programme – we will need to take measures to ensure all patients are safe when they come to get their vaccination, and we will need to ensure people, particularly in at risk groups, are confident in doing so. If a COVID-19 vaccination is available for use then this will also need to be factored in. The college has developed guidance for GP practices to support them to deliver mass vaccination programmes efficiently and safely, whilst COVID-19 remains a threat.'
BMA GP committee chair Dr Richard Vautrey said: 'While GP practices continue to work hard to do all they can to support patients during the COVID-19 pandemic they are also very aware of the vital importance this winter of protecting as many people as possible from the serious impact of influenza.
'It’s good to see the government respond to our call to expand the flu programme but we will also need their support to encourage as many eligible people as possible to get a flu immunisation, as well as ensure vaccination supplies reach practices quickly and in sufficient quantities to meet patient need.'
Health and social care secretary Matt Hancock said: 'It's mission critical that we pull out all the stops to get ready for winter, and the prime minister has already announced £3bn to protect the NHS.
'We are now taking another important step to help protect the wider public by giving the flu vaccination to more people than ever before. This will be the biggest flu vaccination programme in history, and will help protect our NHS as we head into winter.'
NHS England medical director for primary care Dr Nikki Kanani said: 'Getting a free NHS flu vaccination is a quick and simple way that people can help to save lives and reduce pressure on our hardworking frontline staff this winter.
'GPs, nurses, community pharmacists and others will be going to great lengths this year to give this vital protection to millions more people in a safe and convenient way, so when the time comes I would urge everyone invited for a flu vaccination to get it as soon as possible.'