By Greta Rosen Fondahn and Helen Reid

STOCKHOLM, June 25 (Reuters) - Fashion retailer H&M HMb.ST is adapting its clothes and marketing calendar to account for longer, hotter summers, CEO Daniel Erver said on Thursday, as a deadly heatwave gripped Europe for the fourth day.

H&M is designing autumnal collections out of lighter materials to attract shoppers when temperatures stay scorching until the end of September, Erver told Reuters.

"We see that the tendency is that summer runs longer," he said in an interview. "If August is 35 degrees (Celsius), or September even is 35 degrees, you still want to update your wardrobe."

Unseasonably warm weather stretching into the back-to-school season, traditionally when stores start selling jackets and coats, has played havoc with retailers' carefully planned sourcing and marketing schedules and led to overstocks and discounting in recent years.

H&M needs to make sure it has a material mix that works when it is very warm outside, Erver said, particularly in southern Europe, Asia and southern parts of the United States.

SHORTS AND TANK TOPS

Retailers globally have to adapt to a hotter and more unpredictable climate as global warming disrupts supply chains and impacts consumer spending.

"Of course, when a heatwave comes, shorts become important, tank tops, linen, swimwear," Erver said. "In the short time period, the consumers are very affected by how the weather changes."

But he was cautious about extrapolating any broader sales impact from this, saying the fluctuations were short term.

Overall, H&M expects its June sales to be flat compared to last year, it said when reporting weaker-than-expected results for the second quarter on Thursday.

H&M is trying to boost sluggish sales growth, but results have been slow to show, as it struggles to compete with ultra-cheap online retailers like Shein and Inditex's ITX.MC Zara at the upmarket end of fast fashion.