The new smaller packs weigh 122 grams rather than 150 grams as before.
McVities, the makers of the snacks, have cut the recommended retail price from £1.15 to 99p to reflect the changed size.
Twin packs, which now have 20 cakes rather than 24, are now £1.89 rather than £2.19.
This is unlike other companies who have also slashed the size of their food but declined to change the price.
McVities, which is owned by Turkish group Pladis, said in a statement: “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.”
It is not clear why, if price is not the driver of this decision, McVitie’s have reduced the size of Jaffa Cakes.
But for whatever reason, they are just the latest in a long line of producers to slash sizes.
The Office for National Statistics revealed in July as many as 2,529 products had shrunk in size over the past five years, but were being sold for the same price.
Everything from confectionary to orange juice and even toilet roll had shrunk.
Consumers reacted angrily to the Jaffa Cakes news this morning, sharing their feelings on Twitter.
One wrote: “The news of @McVities making the packs of Jaffa cakes smaller has ruined my day; whoever made this decision should be ashamed #jaffascam”.
Another fumed: “It takes a lot to outrage me, but just read that @McVities have skunk a packet of Jaffa Cakes from 12 to 10 for same price!#jaffacakegate”.
Many compared the reduction in size to the shrinking of Toblerone late last year.
The iconic chocolate bar is distinctive in its peaking pyramid-like shapes filled with chunks of nougat.
But makers Mondelez International decided in November 2016 to widen the gap between the triangular pieces, thereby narrowing the amount of chocolate.