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Denmark international Dalsgaard, who became the first Brentford player to play in the World Cup last year, will stay at Griffin Park until the end of the 2020/21 season.
Head coach Thomas Frank said: "I am very pleased that Henrik has agreed a new contract. He is a very important part of what we are doing here and it is great that he is continuing on the club's journey as we progress.
"His performances this season have been excellent and he has been one of our best players. He is also one of the key people in the dressing room and makes others better with his personality and communication.
"Henrik is one of our more experienced players but he is still adding to his game. It is great for me to see as a coach that someone who has played hundreds of games, dozens of internationals and in a World Cup can still add layers to his game. This is great news for the club."
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