Strike action will begin today by staff at Petrobras in Brazil.
The move will be taken at the state-owned firms Campos Basin assets which is the largest oil-producing area in the country.
The Campos Basin produces around 60% of Brazilian oil.
Staff rejected a proposal by Petrobras of a 6% rise in wages.
Questions have been raised by unions over the proposed changes to contracts which would allow the firm to reduce the number of hours worked.