A union representing more than 10,000 Starbucks employees said a strike that began Friday had expanded to include more than 300 stores on Tuesday, the planned final day of a five-day walkout.

Workers in about 10 to 15 stores across the Los Angeles, Seattle and Chicago areas kicked off the strike, which spread to a similar number of stores in three additional metropolitan areas each day. By Monday, baristas in more than 50 stores in cities like Boston, Dallas, Denver, New York and Philadelphia did not report for work.

The strike spread to additional cities on Tuesday, union representatives said, as well as to additional stores in cities where some baristas were already on strike, potentially dampening business at one of the company’s busiest times of the year. Closed stores are expected to reopen on Wednesday, when workers have said they would return to work.

A typical company-owned Starbucks store in the United States brings in sales of about $4,000 to $8,000 during a normal business day, according to a former company official and a review of company financial filings. The figures tend to be higher in December.

A Starbucks spokesman said on Monday he could not confirm the sales figure.

The union, Workers United, represents Starbucks employees at more than 500 company-owned stores across the country, about 5 percent of the U.S. total. It said it called the strike because Starbucks had yet to resolve more than 150 unfair-labor-practice charges on issues like retaliatory firings and cuts to hours, and because Starbucks had not offered a substantial wage increase during contract negotiations.

In the most recent bargaining session, which took place last week, the company offered to guarantee baristas a wage increase of at least 1.5 percent a year. If the company raised wages for all retail employees more than that — as it did with a recently announced 2 percent increase for the least experienced workers — unionized baristas would get the higher amount.