Line two baking trays with non stick baking parchment.
Cream the butter or margarine with the caster sugar until very soft, light and fluffy.
Sift the flour and fold it into the creamed mixture.
Whisk the egg whites until stiff, but not too dry and gently fold them into the mixture - the resulting paste should be just stiff enough to pipe.
Spoon the mixture into a large piping bag fitted with a 5 mm/ 1/4 in plain nozzle.
Pipe 7.5 cm/3 in strips of the mixture on to the prepared baking trays, spacing them a little apart to allow room for the biscuits to spread.
Bake in a moderately hot oven (200 C, 400 F, gas 6) for about 8 minutes, or until the edges of each biscuit are light golden.
Allow the biscuits to cool on the trays for a few seconds, then transfer them to a wire rack to cool completely.