Second class post is 'same as first class' Mail.
The note to mail centre and delivery office bosses said: "Second class traffic posted Friday and Saturday will be processed with first class sent Saturday."
This change was made last July and may have cost customers pounds 4million, as first class stamps cost 46p and second class are 36p.
The Forum of Private Business said: "People have been cheated." Shadow Business Secretary Chuka Umunna said: "This will undermine trust."
Royal Mail said: "First class mail continues to be our priority on Saturday and every other day."