Rules on wearing face coverings were lifted in England in July, along with most other Covid restrictions.
But as cases rise the government has faced some criticism for not reintroducing rules requiring masks in certain places.
How many people are wearing masks?
The proportion of people wearing masks has steadily declined.
Between 18 July and 17 October, the percentage of people in England, Scotland and Wales who said they had worn a face covering outside their home in the past seven days dropped 13 percentage points, according to a survey by the Office for National Statistics (ONS).
Of respondents over the age of 16, 95% said they had worn a mask in the week before remaining social distancing restrictions were lifted. By mid-October, this was down to 82%.
A similar trend was found by the polling company YouGov.
Between 15 July and 22 September there was an 12-point drop (from 72% to 60%) in the number of people in the UK who said they were "wearing a face mask when in public places".
Both surveys relied on people saying whether or not they had worn masks. It is possible some people said they done so when they had not.
Older people were more likely to support mask wearing, according to the ONS.
Among over-70s,92% thought wearing a mask was important to slow the spread of Covid. However, only 80% of 16-29 year-olds thought the same.
What are the rules in England?
Venues and businesses are allowed to have their own rules, and passengers can be refused entry on Transport for London services if they refuse to wear a mask without a reasonable excuse.
The official government advice says the current approach in England "enables personal risk-based judgements".
It says: "We expect and recommend that members of the public continue to wear face coverings in crowded and enclosed spaces where you come into contact with people you don't normally meet. For example, on public transport. You should use your judgement in deciding where you should wear one."
What about the rest of the UK?
The proportion of people who told the ONS they had worn a mask has been higher in Scotland and Wales than England.
In the most recent survey, 93% of people in Scotland and 97% of people in Wales said they had worn a mask outside their home in the previous week, compared with 80% in England.
This could be partly because of stricter rules in Wales and Scotland.
Ministers in Scotland recently decided not to scrap rules requiring secondary school pupils and staff to wear masks.
Masks must still be worn in shops, places of worship and on public transport - as well as pubs and restaurants when not seated.
In Wales, masks are still legally required on public transport and in all public indoor areas apart from pubs and restaurants
In Northern Ireland face coverings are no longer compulsory in places of worship, or for students in school classrooms, but they must be worn on public transport and in shops and hospitality venues.
What have ministers said?
Health Secretary Sajid Javid warned on Wednesday that the reintroduction of Covid restrictions in England would be more likely if people "don't wear masks when they really should." He said this included "really crowded" places and those "with lots of people that they don't normally hang out with".
Medical groups have been calling for the reintroduction of rules to force people to wear masks. The British Medical Association accused the government of being "wilfully negligent". But ministers have resisted making any changes to the guidance or rules.
Health Minister Maggie Throup told MPs: "We believe that people can make informed choices. And I think as people see the levels [of cases] rising, then they will look at the guidance again and perhaps make decision to wear their face coverings in more venues."
This claim is not currently reflected in the ONS research. Between the start of August and 16 October, the weekly number of Covid cases rose from 180,964 to 304,862. However, the proportion of people telling the ONS they had worn masks while shopping in the previous seven days dropped from 92% to 82%.
Why isn't the government reintroducing rules for masks?
Some people fear forcing people to wear masks could have a negative impact on the economy.
The government was warned that keeping compulsory face masks could lead to £4bn in lost revenues for the hospitality and events sector, the i reported in July.
But another reason for the government's reluctance could be more political.
While Mr Javid has been encouraging people to wear masks in crowded places, most MPs from his party have been sitting on the packed green benches in the House of Commons without masks.
Introducing rules on masks risks angering some Conservative MPs.
On Thursday, Labour's shadow Commons leader Thangam Debbonaire said Conservatives should wear them to set the "best example to the public".
In reply, Commons leader Jacob Rees-Mogg said many Labour MPs had been pictured without masks at the the party's recent annual conference in Brighton.
And he claimed they were more likely to cover up "when there are television cameras around".