Postal workers are holding a series of strikes, delaying letter and parcel deliveries.
It means the suggested last dates for sending Christmas post have been brought forward.
When are the next strikes?
A wave of strikes has been called for 9, 11, 14, 15, 23 and 24 December - some of the busiest days for pre-Christmas deliveries.
Postal workers belonging to the Communication Workers Union started their industrial action in August.
How will the strikes affect deliveries?
On strike days the Royal Mail says it will not be able to deliver first and second class letters.
However, it will deliver as many parcels and Special Delivery letters as possible.
The timescale commitment offered on Special Delivery Guaranteed items will be suspended the day before any strike.
People will not be able to claim compensation for late deliveries.
Royal Mail delivery offices and customer service points will be closed. Letters will not be collected from post boxes.
Royal Mail has hired thousands of agency workers to help process the backlog.
The Post Office is a separate business, and post offices will be open, although some services, such as posting letters, may be affected. Other services such as bill payments and banking will run as normal.
When do I have to send Christmas post?
The last posting days for delivery before Christmas have come forward.
They are now 12 December (instead of 19 December) for second class post.
And 16 December (instead of 21 December) for first class.
Source: BBC News