But the European Union and UK are planning to relax the previously strict rules.
Tesco said the 300g loaf was aimed at the increasing numbers of single householders.
Also available will be a family size “weekender” loaf weighing in at 1,000g and a loaf for a small family at 600g.
Said Tesco in-store bakery spokesman Simon Holt: “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.
“But more importantly this historic move will be heralded as the best thing since sliced bread for not only the millions of single householders wanting smaller loaves but also larger families wanting something to last longer.
“We believe that having a range of different 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.”
The new sized single household 300g loaf is to be initially trialled at Tesco’s Kensington store while the 600g and 1,000g loaves will be trialled in stores around the country.
Prescribed one pound quantities for bread were established by the Bread Acts of 1822 and 1836 which were then changed into metric measures in modern times.
The 300g loaf will cost 49p, the 600g will cost 89p while the 1,000g bloomer will cost £1.
Warburtons has also introduced a 600g loaf.
