The decision was taken at an emergency meeting of Gorkha Janmukti Morcha's central committee convened by its supremo Bimal Gurung at an undisclosed location near the Darjeeling- Sikkim border.
Tamang was removed from the post of GJM convener last night for his decision to suspend the Darjeeling shutdown till September 12.
The meeting also decided that the GJM will attend the September 12 talks with the state government scheduled to be held in north Bengal.
Reacting to his expulsion, Tamang said, "This is completely undemocratic. I am a founding member of the GJM. They can't expel me like this. I will seek legal opinion against this decision.
"I am not going to form a separate party," he added.