More half of England’s population are set to be subject to the tightest set of restrictions from 00:01 GMT Wednesday.
Health Secretary Matt Hancock announced the increased measures for London, parts of Essex and parts of Hertfordshire.
There are exceptions to the rules in all tiers for childcare and support bubbles.
BBC Health correspondent Laura Foster runs through the list of things you can and cannot do in tier three.
Video by Laura Foster, Mattea Bubalo and Mel Lou.
Find out more: What Covid tier is your area in?