3M Co.’s pending acquisition of Avery Dennison Corp.’s office products unit may face an anti-trust lawsuit by the U.S. Justice Department, sources told Bloomberg News late Tuesday.

The Pasadena labeling and signage company signed a definitive agreement in January to sell its office and consumer products business to the St. Paul, Minn. technology and consumer products giant for $550 million in cash. The deal would enable Avery to concentrate on its adhesive paper, label and sign business as the office supplies industry continues to struggle post-recession.

Avery’s Brea-based office products business makes Marks-A-Lot pens, binders and other office materials, and would complement 3M’s line of Post-It notes, Scotch Tape and similar products.

3M had planned to complete the purchase in the second half of this year. The Bloomberg report said the Justice Department, which declined to comment for the story, could move to block the deal as soon as the end of this week.

The report hit the wires just before the markets closed. Avery Dennison shares closed down $1.63, or 5 percent, to $29.60 in the Nasdaq.