Charlie Gilmour was one of thousands who demonstrated in December against rising university tuition fees and was among a group that broke away from the main demonstration and attacked a convoy carrying Prince Charles and his wife, Camilla. At Gilmour's trial earlier this year, a judge said Gilmour had jumped on the hood of a Jaguar in the royal motorcade and thrown a garbage can at the car.
Gilmour also kicked a store window, stole the leg of a mannequin and was photographed hanging from a Union flag on the Cenotaph, a memorial to British war dead.
The 21-year-old Cambridge University student, who has been in jail since July, pleaded guilty to violent disorder but challenged the length of his sentence.
But three appeals judges said the sentence was neither "manifestly excessive (nor) wrong in principle."
"We do not believe that violence in this context and of the kind displayed by this defendant can normally be met by other than significant sentences of immediate custody even for those of otherwise good character," said one of the three, Anthony Hughes.