(The Hill) — Starbucks has violated labor laws by refusing to recognize unionizers at a Seattle, Wash., store and must sit down for negotiations with the representatives, the National Labor Relations Board (NLRB) said Wednesday. The Starbucks Reserve Roastery store officially voted to unionize in April, and the election was certified by the NLRB in May.

But since July, Starbucks has continued to challenge the election without producing any new evidence. The store has also refused to negotiate with and recognize the union, violating labor laws, the NLRB said.

The federal agency ordered Starbucks to cease and desist its failure to recognize the union and to bargain with union representatives.

Within 21 days of receiving the notice, Starbucks must file a form with local NLRB officials attesting to the steps it has taken to comply with the order.

Employees at the Starbucks Reserve Roastery in Seattle’s Capitol Hill neighborhood voted 38-27 to join Workers United, which is affiliated with Service Employees International Union.

The store joined more than 250 Starbucks locations that have unionized in the past couple of years, part of a new movement spreading rapidly at stores across the U.S.

Starbucks has pushed back aggressively to the efforts, and union representatives have accused the coffee giant of union busting.

A store in Memphis, Tenn., was ordered to reinstate seven employees in August after a judge found the company illegally retaliated against them for joining a union.

Earlier this month, Starbucks employees at more than 100 stores went on strike on Red Cup Day, typically one of the busiest times of the year when the company hands out free reusable cups to customers for the holidays.

Starbucks CEO Howard Schultz, who returned to lead the corporation in April but is stepping down from the top spot next year, has called the unionization efforts a “new outside force that’s trying desperately to disrupt our company.”