Zalando has posted surging half-year sales as its customer base and the average number of orders they place per year both increased.

The German company said revenues spiked 21.5% to €2.08bn (£1.8bn) during the first half of 2017, following a 20.1% jump in second quarter sales to €1.1bn (£1bn).

Despite strong sales growth, adjusted EBIT remained flat year-on-year at €102.1m (£92.3m).

Zalando reported it expects full-year revenue growth to fall within the ‘upper half’ of the 20% to 25% guided range, with the adjusted EBITA margin in the ‘lower half’ of the 5% to 6% forecast.

During the second quarter of the year, Zalando’s active customer base jumped 800,000 to 21.2 million, compared to the previous quarter.

The Berlin-based etailer said this represented its strongest increase in shopper numbers since the fourth quarter of 2015.

Zalando’s customers are now ordering from the business an average of 3.7 times per year, which it hailed as a sign of ‘strengthened customer loyalty.’

The online operator plans to expand its European fulfilment network with two large warehouses in Poland and Italy.

It already operates three sites of a similar size in Germany and aims to open the Polish distribution centre in Gryfino during its current quarter.

Co-chief executive Rubin Ritter said: “We firmly believe that growth is the right strategy to increase the value of our business. Our updated guidance reflects our focus on growth at solid profitability levels.

“It is in our DNA to continuously evaluate additional investment opportunities, test ideas, and then start scaling them.

“This can range from assortment additions to our recently launched customer loyalty programme Zalando Zet.”