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It was announced earlier this week that van der Mark would be leaving the works Yamaha team at the end of this season, the Dutch rider having been with the Crescent-run squad since 2017.
The three-time WSBK race winner's expected to move to BMW was confirmed on Thursday morning, which will come at the expense of either Tom Sykes or Eugene Laverty.
Team boss Shaun Muir said: “We are delighted to welcome Michael to the BMW Motorrad WorldSBK team. He brings a wealth of experience and is one of the youngest, most talented riders on the WorldSBK grid.
"Securing Michael's services emphasis the focus and determination we have to compete for top honours, and we will give Michael every chance to reach that goal."
BMW Motorrad's motorsport director Marc Bongers added: "We are very pleased about the commitment of Michael, who has developed very strongly in the World Superbike Championship in recent years.
"We are convinced that Michael is an enrichment for our project, and together, in combination with the strong package we can offer him, we can reach the next higher level in terms of results."
Van der Mark becomes the fifth rider to secure a deal for the 2021 WSBK season, joining Kawasaki pair Jonathan Rea and Alex Lowes, Ducati's Scott Redding and van der Mark's current Yamaha teammate Toprak Razgatioglu.
Razgatioglu won the opening race of the year at Phillip Island for Yamaha in March and lies third in the current standings, two places ahead of van der Mark.
Van der Mark to leave Yamaha WSBK squad after 2020
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