STOCKHOLM, Sweden- Zalando, Europe's biggest online fashion retailer, saw growth in unique visitors to its website slow to single digits in the first two months of the second quarter, data showed on Tuesday ahead of a sales update expected later in the week.

Data from U.S. Internet analytics firm ComScore showed the number of unique visits to the retailer's website from personal computers growing 6 percent in April from the previous year and 4 percent in May.

That follows double-digit increases seen throughout the first quarter when unique visits rose 17 percent in both January and February, and 11 percent in March.

The data, however, does not include key mobile data, which now makes up over 35 percent Zalando's traffic.

Zalando, which competes with British online fashion retailer ASOS, has previously said it has no plans to expand to new markets this year and is spending less on marketing, meaning the pace of growth in unique users should naturally slow.

Shares in Swedish investment firm Kinnevik, which has a 36 percent stake in the German company and which reports quarterly earnings on Friday, have been sensitive to news flow on Zalando.

The stock slid in the first three months of the year, partly over worries that growth at the online retailer was slowing, but recovered after Zalando's strong first-quarter results.

IPO-candidate Zalando saw sales in the first quarter rise 35 percent to 501 million euros, slightly down from a 36 percent growth rate in the previous quarter, while the group's core margin improved significantly in the period.

Kinnevik shares traded 0.6 percent lower at 282.40 crowns at 0941 GMT compared with a wider Stockholm bourse which was down 0.3 percent.