The National Labor Relations Board and the Workers United union did not reply immediately to requests for comment.

The coffee chain said NLRB agents allegedly helped Workers United win elections by manipulating the voting process and then collaborated to cover up the behavior, the company said in a letter to NLRB Chairman Lauren McFerran and General Counsel Jennifer Abruzzo, according to a copy seen by Reuters.

A year ago, there were no unionized corporate-owned Starbucks locations in the United States. Now, employees at 216 cafes have voted to join the union, while workers at 46 have voted against unionizing.

Starbucks Corp's allegations come amid a growing unionization drive among U.S. retailers, including Amazon.com Inc, Chipotle Mexican Grill Inc, Lululemon Athletica Inc and grocer Trader Joe's.

According to Starbucks, the alleged misconduct, which the company says it learned from a whistleblower who is a long-time NLRB employee, took place during a Kansas City, Kansas union election, but is believed to have taken place in other regions as well.

"If the NLRB does not respond by investigating and remedying these types of actions, we do not see how the Board can represent itself as a neutral agency adjudicating unfair labor practice disputes - and elections - in a manner that is fair, honest, and proper, without the appearance of impropriety," Starbucks said in its letter.