The meeting is "essential to advance the final peace process", Raymond Kendall, an expert with the International Contact Group, said in the French city of Bayonne on Monday.
Kendall is a former head of the French-based international police network Interpol.
The meeting will take place in the Villa Arnage, built by Cyrano de Bergerac author Edmond Rostand in Cambo-les-Bains in the French Basque Country.
Eta, which was founded in 1959 to fight for the independence of the Basque Country and Navarre, renounced armed action in 2011.
In April last year it revealed the location of eight arms dumps in south-west France to the French authorities, a move that was welcomed by Paris but judged insufficient by Madrid.
Apology to victims
Last Friday it issued an apology to victims who were not involved in the 43-year conflict.
The right-wing government of Mariano Rajoy has refused to negotiate with Eta but hailed a victory for the "rule of law" after the apology.
Eta is blamed for the deaths of 829 people and some of its members were victims of extrajudicial killings and torture.
About 300 of its members are in prison in France, Spain and Portugal and up to 100 are on the run, according to a prisoners' support group.
It is expected to officially disband on the weekend of 5-6 May.