The 12 metre-tall tree comes complete with a Lego star and 12,000 Lego baubles.
The Lego Christmas tree was created by Duncan Titmarsh from the professional Lego building company Bright Bricks.
It is made from 600,000 Lego bricks and has 172 branches and was built by kids from Edith Neville Primay School in Camden and Copenhagen Primary School in Islington along with the Harpenden Explorer Scouts.
David Buxbaum, marketing director at Lego said: "The Lego brand is all about play and creativity and Christmas is the time for celebration and fun."
David Titchmarsh adds the star to the tree.
He added: "This magnificent Christmas tree is a shining example of these values and shows that with Lego bricks you can let your imagination run wild and build anything. We hope that it will work to remind people that Lego toys are the Christmas gift that keeps on giving."
The tree will remain in St Pancras Station until January 3rd.