See ESC right:

Collective bargaining and agreements

Relevant ICESCR Articles:

08(1)(a): The States Parties to the present Covenant undertake to ensure:
(a) The right of everyone to form trade unions and join the trade union of his choice, subject only to the rules of the organization concerned, for the promotion and protection of his economic and social interests. No restrictions may be placed on the exercise of this right other than those prescribed by law and which are necessary in a democratic society in the interests of national security or public order or for the protection of the rights and freedoms of others;