More than a thousand unionised Starbucks baristas have walked off the job in more than 40 cities across the United States as negotiations have stalled between the company and the union, Starbucks Workers United.Workers at 65 stores began an open-ended strike on Thursday, coinciding with the Seattle, Washington-based coffee shop chain’s Red Cup Day sales event, when customers who order a holiday-themed beverage can receive a free reusable cup with their purchase.Recommended Stories list of 4 itemsend of listThe event typically drives higher traffic to Starbucks stores.The coffeeshop chain, which has more than 18,000 stores across the US and Canada, says that the walkouts have caused limited impact.More stores could soon join the strike. Starbucks Workers United represents roughly 550 stores around the US. Combined, this strike could be the largest in the history of the coffeeshop chain.Stores in cities including Seattle, New York, Philadelphia, Dallas, Austin and Portland will join the work stoppage, it said. Some locations had already shut down for the day, a union spokesperson told journalists on a media call.In an Instagram post on Thursday, the union called on consumers not to shop at any Starbucks location “today and beyond” ahead of a nationwide rally slated to begin at 4pm local time for each location.The union has filed more than 1,000 charges to the National Labor Relations Board for alleged unfair labour practices such as fi …