The whole of Scotland could return to the top level of Covid-19 restrictions if people do not follow the latest travel rules, it has been warned.
A new five-tier system of measures means different curbs will be in force in different parts of Scotland.
People are being urged not to travel into or out of council areas where there are level three restrictions.
First Minister Nicola Sturgeon said if people did not abide by advice, the new model would become "unsustainable".
She said she could not rule out a move back to nationwide measures, and that this could involve level four restrictions - which are close to a full lockdown.
The new five-level system - running from zero to four - will come into force in Scotland from Monday, 2 November.
No areas have been placed in the highest level, although it was considered for North and South Lanarkshire.
Much of the central belt and the city of Dundee have been placed in level three, with most other parts of Scotland in level two.
Until now the advice for people living under tighter restrictions in the central belt has been not to travel outside their own health board area.
But Ms Sturgeon said that from Monday, the guidance would apply at council level.
That means people living under level three rules should not travel outside their own local authority area for anything other than "essential" reasons.
This can include work, education, outdoor exercise, healthcare or caring responsibilities, and essential shopping.
People living in level one or two areas are also being told not to travel into a level three area, except for essential purposes.
Ms Sturgeon said these rules were not being put down in law initially because police could not check everyone's journey - although this would be kept "under review".
Ms Sturgeon also said people should not travel between Scotland and other parts of the UK with high levels of the virus "unless they really need to do so".
The first minister said sticking to the advice was "essential".
She said: "If people don't abide by the travel advice, the virus will spread from high to lower prevalence areas, and a differentiated approach will become unsustainable.
"In these circumstances, we would have to return to national restrictions."
Ms Sturgeon urged people to follow the advice "to keep everyone safe and to allow us to continue with a proportionate response to wider restrictions".
She said progress was being made in Scotland - but that the situation was "fragile".
"So I cannot rule out a move back to nationwide restrictions in the next few weeks, including at level four," she said.
"That could happen if, for example, cases in parts of the county start to rise faster again, to the extent that controlling spread with travel restrictions will not be effective.
"Or it could happen if pressure on the NHS risks breaching capacity - not just at a local level - but overall."
She said everyone needed to play their part to avoid that happening.
In a couple of weeks, what level could your area be in?
The Scottish government has issued more information about how it chose the levels for each council. It has also set out forecasts for the coming weeks.
Taking into account current positive cases and relative population size, Imperial College modelling has suggested that;
- seven Scottish authorities - including East Renfrewshire, South and North Lanarkshire - would move into Level 4 in the week beginning 8 November
- eight other councils - including Edinburgh and Aberdeen - would move into the lowest category, Level 0
- eight areas - including Inverclyde and Stirling - would be moved from Level 3 to Level 2
- and, although most of the central belt is currently in Level 3, based on this modelling just five councils would be in that category - Clackmannanshire; Dundee City; Glasgow City; Renfrewshire and South Ayrshire
Imperial College modellers set a percentage confidence level against the forecasts. To find out more go to the Scottish government website