In an address to the nation on 4 January, prime minister Boris Johnson said the rapid spread of COVID-19 and in particular the new variant identified late last year had been 'frustrating and alarming'.
NHS hospitals are under more pressure than at any time since the start of the pandemic, he said, with almost 27,000 COVID-19 patients in hospital. More than 350,000 people have tested positive over the past week in England alone.
Mr Johnson said deaths had risen 'by 20% over the last week and will sadly rise further'.
The Scottish government has also announced a national lockdown, while Northern Ireland and Wales are already in lockdowns.
GP vaccination sites will begin administering doses of the Oxford/Astra Zeneca vaccine - the second vaccine approved for use in the UK - from this week, and reports suggest that including available doses of the Pfizer/BioNTech vaccine as many as a million doses may be available this week.
Around 1m doses were delivered to patients in England by the end of 2020, the government has confirmed - and Mr Johnson said the NHS would target vaccination of the top four groups prioritised by the Joint Committee on Vaccination and Immunisation (JCVI) by mid-February.
This would cover all people over 70 years of age, care home residents and staff, health and care staff, and patients who are clinically extremely vulnerable.
In England there are 7.8m patients over 70 registered with GP practices, around 2m on shielding lists, an estimated 400,000 in care homes, around 1.3m NHS staff and around 1.5m social care staff - although there is crossover between these figures.
Minister for COVID vaccine delivery Nadhim Zahawi said on Twitter that the government was aiming to deliver 13.9m vaccines UK-wide by mid-February.
Cabinet Office minister Michael Gove, meanwhile, has suggested lockdown measures in England could remain in place until March.
Shielding measures for patients who are clinically extremely vulnerable have been reinstated across England.
The shift to a full lockdown in England and Scotland came after Mr Johnson said on Monday that there was 'no question' further restrictions were needed to control the pandemic.
House of Commons health select committee chair Jeremy Hunt said on Monday that pressure on hospitals was now 'off-the-scale worse' than the four winter crises seen in the NHS during his five years as health secretary.
BMA chair Dr Chaand Nagpaul said: 'It’s clear that we need a major intervention to bring down the spread of this virus, especially the new more aggressive variant, given that the NHS in on the brink - currently facing exponential demand for care beyond what can be supplied in many places.
'Hospitals are stretched to breaking point, with doctors reporting unbearable workloads as they take on more COVID-19 admissions alongside the growing backlog of people who need other, non-COVID care. Doctors are desperate, with some even comparing their working environment to a warzone as wards overflow, waiting lists grow, and ambulances queue outside hospitals because there are now so many people with COVID-19.
'Without these tougher measures, the further this virus will spread, potentially taking more lives and damaging the NHS beyond repair. This lockdown is temporary, but the grief and loss suffered by those [who lose family members] because of COVID-19 is enduring - and a stark reminder of our collective responsibility to ensure that it is not this virus that wins, but us.'