In a large mixing bowl, combine the water, levain starter and flour. Stir in the salt and sugar and mix until the dough pulls away from the sides of the bowl. The dough will be very sticky - leave it for around 20 minutes to allow the flour to absorb the water. This will make it easier to knead.
Knead the dough for around 10 minutes on a lightly greased surface. The dough will be sticky and you can add more flour if necessary but try not to add too much.
Place the dough into an oiled bowl and place it in a warm spot until doubled in size, for around 2 hours.
After 2 hours, give the dough a few folds, and divide into 2 pieces. Shape each piece into a long loaf then place on a baking tray lined with baking parchment. Cover with a floured tea towel and let them rise once more for 1-2 hours until puffy.
Preheat the oven to 230C | Gas Mark 8.
Using a sharp knife, make three diagonal slashes in each loaf.
Bake in the middle of the oven for about 22-26 minutes or until golden.
Cool completely on a wire rack.