Tesco is introducing three loaves in non-standard sizes ahead of anticipated changes to EU laws.
The supermarket giant will sell a 300g loaf aimed at single households, a family-sized "weekender" loaf of 1,000g and a 600g loaf for small families.
Current laws stipulate bread sold in the UK above 300g must be sold in 400g sizes, or multiples of 400g.
But Tesco said it was introducing the new sizes in anticipation of changes to EU law.
ISB bakery buyer Simon Holt said: "The end of restrictions on the size of a traditional standard loaf is a huge milestone for the British bakery industry and one that takes into account important social changes.
He added: "Customers have told us that they want a really small loaf as well as an extra-large one, so this is what we have done. We believe that having a range of bread sizes will help towards reducing food waste in this country and will finally bring Britain in line with bread sizes across the rest of Europe."
Tesco will trial the smaller loaf at its store in Kensington, London. The 600g and 1000g loaves will be trialled at stores across the UK.