The UK's COVID-19 vaccination campaign has so far been focused on the top two priority groups identified by the Joint Committee on Vaccination and Immunisation (JCVI) - covering care home residents and staff, over-80s and health and care workers.
From today the programme expands to include the third and fourth priority groups, covering patients over 70 years old and those who are clinically extremely vulnerable.
However, the government says the first two groups 'remain the priority', and that only vaccination sites with enough supply and capacity for vaccinating further people should offer vaccinations to the next two cohorts.
The move is intended to allow areas that have already provided a first dose of vaccine to most care home patients, health and care staff and over-80s to 'keep up the momentum and start vaccinating further at-risk people'.
The government hopes to vaccinate everyone in the top four cohorts, a total of 15m people in the UK, by mid-February.
More than 4m people have so far received a first dose of vaccine, according to the latest government figures. This includes more than half of patients aged over 80 and half of elderly care home residents, the government has said - with more than quarter of a million doses a day now being administered.
Health and social care secretary Matt Hancock said: 'Now that more than half of all over-80s have had their jab, we can begin vaccinating the next most vulnerable groups. Where an area has already reached the vast majority of groups 1-2, they can now start opening up the programme to groups 3-4.
'We are working day and night to make sure everyone who is 70 and over, our health and social care workers and the clinically extremely vulnerable are offered the vaccine by the middle of February and our NHS heroes are making huge strides in making this happen.
'This measure does not mean our focus on getting care homes, healthcare staff and those aged 80 and over vaccinated is wavering - it will remain our utmost priority over the coming weeks to reach the rest of these groups.'