JOHANNESBURG (Reuters) - A strike certificate has been issued at South African airline Comair COMJ.J as part of a secondary strike against South African Airways, a spokeswoman for the National Union of Metalworkers South Africa said on Wednesday.

Two unions that called a crippling strike at SAA said on Sunday they would pursue a secondary strike to ensure their demands are met, that they warned could shut down the entire aviation sector.

After a strike certificate is issued, unions have 48 hours to notify employers of any strike action.