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Armstrong confirmed the decision in a statement following the conclusion of the World Rally Championship (WRC) season, saying both had agreed the time was right to pursue different plans.
He described it as a difficult decision but said it presented an opportunity for both to explore new challenges.
The pair enjoyed a number of notable successes together over the past two years, including victories at Rali Ceredigion and Rally Croatia.
Armstrong also highlighted their opportunity to step up to Rally1 machinery in the WRC this season as a particularly special moment in their partnership.
Paying tribute to Byrne, Armstrong thanked his co-driver for his hard work, commitment and effort throughout their time together.
“We’ve shared some fantastic moments along the way, and I wish Shane all the very best for the future,” he said.