The cable outlined Cairo’s warnings that a southern vote for independence in 2011 could have “fatal implications,” including destabilising the Horn of Africa, causing an influx of migrants to Egypt, and hurting Suez Canal revenues.

The plebiscite was promised by the 2005 Comprehensive Peace Agreement, which ended decades of civil war between the north and oil-producing south. It is due to take place on Jan. 9, but preparations are falling behind schedule.

The leaked cable, from the US Embassy in Cairo, said officials asked the US government to “educate” southern leaders on the dangers of separation and encourage them to advocate for unity.

Egypt suggested amending the peace deal to give the south autonomy for ten years before choosing between federation or independence, or delaying the vote for four to six years to develop south Sudan’s “capacity for statehood,” the cable added.