The tier depends on rates of Covid-19 transmission in that local area, and each has respective rules attached, with the most serious restrictions banning socialising with other households both inside and outside.
Although the system has only been in place a matter of weeks, on Monday 26 October, health secretary Matt Hancock said the government would “rule nothing out” when questioned about the introduction of a new - more serious - fourth tier.
Mr Hancock told BBC Breakfast: "We've always said all along that we take nothing off the table… having said that, we have seen the rise in the number of cases has slowed a bit.
"The problem is it's still going up, and while it's still going up we've got to act to get it under control,” he added. "At the moment the three-tier system is what we're working to and it's effective in slowing the growth of this virus but it hasn't brought this curve to a halt."
When the three-tier system was introduced, Steven Powis, medical director of NHS England said the number of Covid hospitalisations was higher than before the UK-wide lockdown in March.
So what could a fourth tier realistically look like and how likely is it to happen before Christmas?
How likely is a tier 4?
On 23 October, the i reported that discussions were underway in Whitehall for a “Plan B” in case the three-tier system was not making a noticeable difference by mid-November, and that options being discussed included a “tier three plus or tier four” or circuit-breaker lockdown.
This echoes Mr Hancock’s statement that extra measures have not yet been ruled out.
But Boris Johnson has already made clear that he would not revert to another nationwide lockdown because of the impact on the economy and people’s mental health prospects. “I don’t want a second national lockdown – I think it would be completely wrong for this country and we are going to do everything in our power to prevent it,” he said in September.
Despite this, medical experts have publicly been sceptical about the ability of the three-tier system to bring the R rate below 1 before Christmas.
On the same night Boris Johnson launched the three-tier system, chief medical officer, Chris Witty, said: "I am very confident that the measures that are currently in place are helping to slow the virus.
“I am not confident - nor is anybody confident - that the Tier 3 proposals for the highest rate, if you did the absolute base case and nothing more, would be enough to get on top of it."
Sir Patrick Vallance later echoed his comments, saying: "And as the chief medical officer said, Tier 3 baseline conditions on their own, almost certainly aren't enough to get the R below one.”
On 13 October, 24 hours after the tier system was published, Scientific Advisory Group on Emergencies (SAGE) published a report, which showed that the government had not followed its advice for a circuit-breaker lockdown three weeks prior.
Although a government spokesperson confirmed Downing Street was still taking advice from SAGE, it showed scientists were pushing for further restrictions behind the scenes.
What would a tier 4 look like?
Tier 4 would become the most serious category in any updated system. We can look to the devolved nations, all of which have currently got stricter measures, as an indication of where we could be headed.
In Scotland there are already five tiers of lockdown - Level 0, 1, 2, 3 and 4. Level 3 is stricter than any measures in England currently as it closes much of hospitality completely - in England, places can remain open as long as they are serving food.
Level 4 in Scotland is “closer to a full lockdown”, according to Nicola Sturgeon, with things like non-essential shops being closed. However, even under level 4 people from up to two households can still meet outdoors and there won’t be a limit on outdoor exercise.
This is currently not in place in Scotland - and is being used as an upper threshold if necessary - but could be a model for England to base a higher tier on.
We can also look to Wales and Northern Ireland, both of which are currently undergoing short “circuit breaker” or “firebreaker” lockdowns.
Wales is currently in a 17-day firebreak lockdown, until 9 November, which means you cannot socialise with other households, cannot travel apart from essential reasons, and restaurants, bars and pubs are all closed. You also cannot purchase “non-essential” items in shops.
Northern Ireland is in a circuit-breaker lockdown, which requires pubs and restaurants to shut unless they offer a takeaway service. Overnight stays in other homes are also banned but places of worship, shops and gyms can stay open.
The existing tier 3 measures, although the strictest across England, still have some obvious freedoms, which could be reversed.For example, restaurants, and pubs and bars that serve food are allowed to stay open. This could be a potential area for change as in the other nations.
And, although tier 3 bans socialising with other households indoors and outdoors - you can still visit places like beaches, parks and forests with another household in a group of six or fewer, as long as you maintain social distancing. This loophole could also be removed.
Although the government has continued to maintain schools and universities are the number one priority for remaining open - Professor Neil Ferguson warned BBC Radio 4 on Sunday that schools might have to be shut to some degree (in Wales this is already happening as part of the firebreak).
“That [banning households mixing] should have a significant effect but as yet we have been unable to see it definitively,” said Prof Ferguson.
“If we go beyond that there is a limit to what we can do in terms of reducing contacts, short of starting to target, for instance, the older years in schools and sixth form colleges where we know older teenagers are able to transmit as adults.”