A Swedish court has ruled that the confectionery firm Mars can no longer advertise its M&M's brand with the lower case lettering "m&m".
The court ruled that the logo is too similar to the single lower case "m" used by the Swedish chocolate covered peanut brand Marabou.
If Mars doesn't appeal it will have to use the capital M&M logo in Sweden starting in July.
Mars said it believed "no confusion exists" between the two chocolates .
"We have always believed no confusion exists between the colourful m&m's brand - one of the world's favourite chocolate products - and the Marabou M Peanut Brand," said Mars in a statement.
"Given the court's decision we will assess the next steps for our beloved brand in Sweden," it added.
Marabou is owned by American food and drinks company Mondelez, which also owns the Cadbury and Toblerone brands.
Until 2009 Mars did not sell its M&M's chocolates in Sweden, honouring an earlier agreement with Marabou, which has been using the lower case "m" on its chocolate bars since the 1960s.
This is not the first chocolate related trademark battle to be fought recently.
In January, Nestle lost its case to trademark the finger shape of its KitKat bars. A British court ruled that a Norwegian bar, called Kvikk Lunsj, was entitled to use the same shape.
Kvikk Lunsj is also owned by Mondelez.