Warm September weather in the United Kingdom might be responsible for a slowdown in online sales, but the overall volume of e-commerce-based deliveries is still up by 10.9% compared with September 2013. Cumulative volume growth is holding at 18.9% for 2014.

According to results of today’s MetaPack Delivery Index, which is compiled by U.K. e-retail association Interactive Media in Retail Group, or IMRG, U.K. retailers will take a record 890 million online orders and deliver 928 million parcels in 2014.

In what IMRG called an encouraging pre-peak season trend, the percentage of on-time deliveries for September increased to 94.30% compared with 93.89% in August.

IMRG says the index confirms what other recent retail studies have suggested, that online retail growth slowed in September, likely because of unseasonably warm weather holding back sales of newly launched autumn and winter apparel. “We would expect the weather to worsen and drive up autumn and winter clothing range sales,” says Andrew Starkey, head of e-logistics at IMRG. “But in order to catch up on a quiet September, it is possible that some online retailers will begin to discount earlier.”

“September’s improving data on delivery performance is encouraging, even though it is fractionally less than the same month last year,” says Angela O’Connell, MetaPack’s marketing and strategy director. O’Connell says there is capacity within the U.K. e-commerce industry to handle the forecasted record Christmas online orders and parcel deliveries. “In particular, the rapid development of in-store and third-party click-and-collect networks will play a key role in providing the additional delivery channels retailers will need this year to satisfy festive shoppers,” she says.

The IMRG MetaPack U.K. Delivery Index was designed to track a range of key metrics to enable retailers to compare performance. The index summarizes data from more than 220 retailers and an average of more than 4.5 million orders in any one month.