There used to be 11 of the caramel filled chocolates - famous for their "Do you love someone enough to give them your last Rolo?" commercials - in a tube. But Nestle has reduced the number to 10 - without cutting the price.
The move was highlighted by the BBC1 consumer programme Watchdog, after a complaint from a viewer.
Nestle defended its action, saying it had cut the packet size instead of raising the recommended retail price of 24p.
Sue Brewer, a mother of two from Oundle, Northampton, complained that Nestle had ruined her chocolate treat. She told Watchdog she normally gave each of her two sons five Rolos and spoiled herself by eating the last one. But the last time she bought a pack, the last Rolo had gone.
"I'm feeling cheated because I didn't find anyone who loved me enough to give me some chocolate at a fair price, or to be honest enough to print 'one less, same price' on the packet."
A Nestle spokeswoman said other firms had raised prices of similar brands and said Rolos were still good value. "The action we have taken has enabled us to maintain the price at a competitive level and we believe Rolo offer excellent value for money," she added.