The chain said volatile trading, with customers only buying new clothes when they need them, meant the warm August "worked against some of our clothing ranges".
Next said it failed to stock enough warm weather "transitional stock" - to sell after its July sale and before the launch of its autumn range.
The retailer revealed the weather blow as it reported 2.2% revenue growth to £1.68 billion in the six months to the end of July, helped by its expanding footprint and online sales. The retailer's flourishing online directory business drove pre-tax profits 8.2% higher to £271.8 million.
Next did not quantify the impact of its stock blunder but said it must also take greater risks on buying fabrics - purchasing key materials in greater quantities earlier in the year.
Anusha Couttigane, fashion consultant at Conlumino, said Next was let down by "flaws in its forward planning".
"This is a planning error that mature high street players should not fall victim to," she said.
Next said that while trading conditions looked set to improve gradually on looser credit to households and businesses, "talk of a full-blown recovery is premature".
The clothing giant defied the tough retail climate by laying out plans for a major land grab over the next five years, by opening another 1.4 million square feet of new space.