Teams in old fashioned skiffs will row up a stretch of the river over the next five days to carry out the annual census of the birds, shouting "all up" when they come across a mute swan and its family of young cygnets.
The ancient ceremony dates back to the 12th century when the English crown first claimed ownership of all mute swans, which have long curved necks, orange beaks and white feathers.At the time they were considered a delicacy and would be served at lavish banquets.
The annual event had to be cancelled last year due to the coronavirus pandemic.
Queen Elizabeth shares ownership of mute swans on the Thames with the old trade associations of the Vintners and Dyers, who also join in the upping.
However, the monarch retains the right of ownership of all unmarked mute swans swimming in open waters, although this is mainly exercised on certain stretches of the Thames.