(Reuters) - OfficeMax Inc (>> OfficeMax Inc) reported lower-than-expected quarterly results on lackluster sales to corporate customers and regular shoppers, and warned of weak sales in the period that includes the all-important back-to-school season.

The second-quarter results, reported on Tuesday, come as OfficeMax awaits regulatory approval for its acquisition by larger rival Office Depot (>> Office Depot Inc), which also reported weaker sales last week.

The companies, which combined would still trail industry leader Staples Inc (>> Staples, Inc.), are under pressure to boost profits and shareholder value. A merger would help them cut costs, consolidate stores and boost their clout with suppliers.

Analysts have long called for consolidation in what they see as a cluttered sector whose sales crumbled during the global financial crisis. Office supply stores are also fighting a battle for relevance, with shoppers increasingly buying their paper, toner and technology online or at mass merchandisers.

OfficeMax's second-quarter net loss was $10.0 million, or 12 cents a share, compared with year-earlier net income of $10.7 million, or 12 cents a share. Excluding special items, it earned 2 cents a share, missing analysts' average estimate of 3 cents, according to Thomson Reuters I/B/E/S.

Sales fell 4.3 percent to $1.53 billion, while analysts expected $1.55 billion.

The third-largest U.S. office supply retailer said sales and operating income margin in the current quarter will be down from a year earlier.

(Reporting by Dhanya Skariachan; Editing by Lisa Von Ahn and John Wallace)

By Dhanya Skariachan

Stocks treated in this article : Staples, Inc., OfficeMax Inc, Office Depot Inc