The objective of this paper is to investigate the yet‐to‐develop assurance market, its links with the mature auditing market, and the role that the Big4 auditing firms (KPMG, EY, PwC, and Deloitte) play in the former. We use data submitted to the Global Reporting Initiative by companies in 18 countries, for the years 2011–2013, in order to obtain a global overview that allows generalization of the results. We find higher levels of disclosure and increased credibility of sustainability reports (SRs) when the financial auditor is a Big4. Companies audited by a Big4 are more likely to assure the SR than those audited by a non‐Big4. Our paper confirms the connection between financial auditor and assurer of SR provider in an international setting: the choice of a Big4 as a financial auditor is a driver for the choice of a Big4 as an assurer provider, suggesting a potential competitive advantage. Copyright © 2017 John Wiley & Sons, Ltd and ERP Environment