Measures are being tightened in the face of rising coronavirus infections, with Boris Johnson warning the UK has reached a "perilous turning point" in its fight against the disease.
Tougher restrictions are being brought in across all four nations, but vary – here, we take a look at the new rules in each country.
Working from home is once again being encouraged, with anyone who can being asked to do so.
People who cannot, such as those working in construction or retail, are being advised they should continue to go to their workplaces.
— UK Prime Minister (@10DowningStreet) September 22, 2020
From Thursday pubs, bars and restaurants must offer table service only and hospitality, leisure, entertainment and tourism businesses will all have to close between 10pm and 5am.
People working in retail, travelling in taxis, and staff and customers in indoor hospitality, except while seated at a table to eat or drink, will have to wear face coverings.
From Monday, a maximum of 15 people will be allowed to attend wedding ceremonies and receptions, but the limit remains at 30 for funerals.
The rule of six, introduced last weekend, that any social gatherings of more than six people are against the law, is being extended to all adult indoor team sports.
Large sporting events, business conferences and exhibitions will not reopen as had been planned from October 1.
The penalties for disobeying the rules will also be greater – failing to wear a mask or breaking the rule of six will see fines doubling to £200 for a first offence.
Businesses which break the rules could be fined up to £10,000 and closed.
Fines of up to £10,000 for people who fail to self-isolate have already been announced.
Downing Street said military support was an option to free up police so they can focus on enforcing the tougher rules.
For people in the shielding category, Mr Johnson said the guidance remains that shielding is not currently needed, unless they are in a local lockdown area.
– Northern Ireland
Northern Ireland has the highest infection rate across the UK and Ireland, and fresh Covid-19 restrictions are to be extended from some specific postcodes to the whole country from 6pm on Tuesday.
Households will no longer be allowed to mix indoors, except for single-person bubbles and certain other exemptions.
No more than six people from two households can meet in a garden.
Pubs which do not serve food, known as wet pubs, are due to open on Wednesday, despite the latest restrictions.
We are at a dangerous crossroads, a resurgent COVID19 is spreading and claiming lives.
We have a window opportunity over the next few weeks that we must seize to minimise the spread of the virus.
We need you all to work in partnership with us to keep people safe and save lives pic.twitter.com/inOjHLaDsS
— Michelle O'Neill (@moneillsf) September 22, 2020
Deputy First Minister Michelle O'Neill said whether to introduce an early closing time for pubs is something ministers would be considering, describing replicating the 10pm curfew being introduced in England as "fair enough" to consider.
First Minister Arlene Foster said a two-week period of lockdown to try to halt the spread of the virus, a so-called circuit breaker, could not be ruled out.
Household mixing indoors will no longer be allowed, with exemptions for those living alone, couples not living together, childcare and tradespeople.
Regulations come into force on Friday but people are being asked to comply from Wednesday.
First Minister Nicola Sturgeon said children under 12 will be exempt from the current limit of six people from two households when meeting outside, and those between 12 and 18 will be able to meet a limit of six others from six households outdoors.
From Friday pubs, bars and restaurants must close at 10pm and further resources will be given to environmental health officials to step up enforcement and inspections, to check that social distancing and other hygiene guidance is being adhered to.
People in Scotland are also being advised against car-sharing, with Ms Sturgeon saying that according to Test and Protect data there is a "significant risk of transmission" in such settings.
She said no decision has been taken yet on a so-called circuit-break in October, and the Scottish Government is "keeping it under review".
She asked people not to book any overseas travel for the half-term break unless it is essential, and to use it as an opportunity to "further limit social interaction".
She said people who were shielding earlier in the year are not at this stage being asked to do so again, but that they should follow the steps outlined for the general population.
First Minister Mark Drakeford indicated people in Wales could be asked to make essential journeys only.
He said "one of the central dilemmas" the country faced was the differing rates of Covid-19 in different areas, with rates of the virus still falling in 10 local authority areas.
"I will want to say something later today about trying to encourage people in Wales only to make those journeys that are really necessary," he told the Senedd.
Mr Drakeford said many of the things being announced by Mr Johnson "we have already done in Wales", such as the encouragement to work from home where possible.
Measures across much of South Wales are due to come into force from 6pm on Tuesday.
The measures, which are already in force across Rhondda Cynon Taf and Caerphilly county borough, will apply in Bridgend, Merthyr Tydfil, Newport and Blaenau Gwent.
Under the new rules people must not enter or leave the areas without a reasonable excuse and are only able to meet with other households outdoors, including members of their extended household.
All licensed premises such as pubs have to close at 11pm.
– How long will these new measures last?
Mr Johnson said that "unless we palpably make progress" we should assume the restrictions he announced on Tuesday will remain in place for "perhaps six months".
Meanwhile Ms Sturgeon said she hoped the measures in Scotland would not have to be in place for that long, saying that while the absence of a game-changer vaccine means the virus will impact people's lives "that doesn't necessarily mean that all of the new restrictions I am announcing today will be in place for six months".
On Monday England's chief medical officer Professor Chris Whitty suggested that science would eventually "ride to our rescue", but "in this period of the next six months, I think we have to realise that we have to take this, collectively, very seriously".