Standard-sized toasters are not big enough for popular artisan loaves - leaving the edges uncooked.
Loaves such as sourdough are too wide to fit into a conventional toaster, the consumer watchdog Which? said.
Its researchers reviewed 78 toasters and found the average Slot Mpo88 is 6in wide and 5in high with a variety of depths.
This means toast lovers are left with no choice but to cut their slice of sourdough - which can be up to 9in wide - in half to avoid undercooking the edges.
Loaves such as sourdough are too wide to fit into a conventional toaster, the consumer watchdog Which?
Which? researchers reviewed 78 toasters and found the average slot is 6in wide and 5in high with a variety of depths
A Which? spokesman said: ‘This isn't the end of the world, but it means you can sometimes end up with a pale white border around the edge of your toast where it hasn't had any heat.'
Alex Waugh, director of the trade association UK Flour Millers, said bakers should ‘make sourdough loaves, or artisan loaves, less wide so they can fit in'.
He suggested grilling the larger slices in the oven instead.
Artisan loaves are made using traditional methods and ingredients as opposed to modern techniques.
They have become increasingly popular - cutting the sales of white-sliced bread by 12 per cent in the five years to 2017, the market researcher firm Mintel says.
It added that the market value of speciality bread and rolls was expected to be £1.1billion in 2020.
Lisa Barber, the Home Products and Service Editor for Which?, said: ‘If you're a bit of a toast perfectionist, there are a handful of toasters that will meet your baked goods needs, whether that be for an artisan bread slice or a bagel.'