Canada, the United States, and Mexico have entered a confidentiality agreement to protect each other's offers during NAFTA negotiations, with a list of rules designed to prevent leaks during the talks.

Each government is prevented from distributing texts, emails, proposals and presentations gathered from the other countries.

There are two exceptions: the material can be shared internally with government officials and externally with the stakeholders governments consult on the negotiations. The materials are to be stamped, "Confidential," and, when not being used, are to be protected in secure locations like locked file cabinets.

The agreement expires four years after negotiations conclude.

The countries can share their own documents with whomever they like, but the agreement forbids them from leaking other countries' materials.

"The policy underlying this approach is to maintain the confidentiality of documents, while at the same time allowing the negotiating parties to develop their negotiating positions, communicate internally and with each other and engage with their public as they consider appropriate in developing and communicating their own positions," said the document.

'Standard' in trade talks

Such non-disclosure agreements are being called standard in trade negotiations.

The agreement was posted online this week by the United States Trade Representative, after being signed last month by the three countries' lead negotiators.