Jaffa Cake fans have been left outraged after boxes of them dropped in size from 12 to 10 per pack.
Despite McVitie's reducing the recommended retail price (RRP), some shops appeared to be selling Jaffa Cakes for the original amount.
While they cited rising production costs as a factor, McVitie's were less clear about the decision over Jaffa Cakes.
Several fans took to Twitter to express dismay at the move and idea of "shrinkflation".
The RRP of a 12-pack of Jaffa Cakes was £1.15, but this has now dropped to 99p.
Twin packs, down from 24 to 20 cakes, have had their RRP reduced from £2.19 to £1.89.
A McVitie's spokeswoman said: "As a company, we have taken the decision to adjust the pack size of our McVitie's Jaffa Cakes products and where we've reduced cake count, the RRP has also been reduced proportionately.
"This applies to all the core grocery channels."
Jaffa Cake eaters described themselves as "upset" at the change.
The McVitie's spokesperson added: "These changes represent the same price per cake, however pricing ultimately remains at the sole discretion of retailers.
"There is no change in the size, shape or weight of individual cakes in the McVitie's Jaffa Cake range."