STORY: The Starbucks strike intensified on Christmas Eve.

Its union estimates that more than 300 U.S. stores would participate throughout the day –

which led to the cessation of work of more than 5,000 workers.

Negotiations between Starbucks and the union were at an impasse due to unresolved issues over wages, staffing and hours, leading to the strike that began Friday.

With a collective bargaining agreement, improved benefits, wages and working conditions could be written down and could not be reduced without negotiation, according to the union’s website.

The Christmas Eve strike is expected to be the biggest ever at the coffee chain, the union says.

A statement posted on Starbuck’s website said it expected a “very limited impact” on overall operations.