ZANZIBAR-- Separatist campaigners on the East African island of Zanzibar have appealed to a regional court to challenge the legality of its union with mainland Tanzania, they said Thursday.

Rashid Salum Adiy, head of the Zanzibar Rights of Freedom and Autonomy (Zarfa) organisation, a group not recognised by the Tanzanian authorities, claims to have the backing of 40,000 supporters for his appeal.

Adiy, who has organised protests in the past denouncing the union between Zanzibar and Tanzania, argues the independence-era union agreement favours the mainland at the expense of the Indian Ocean archipelago.

He has filed an appeal to the East African Court of Justice to challenge the legality of agreement.

"Based on the evidence we have, we argue that this union has no legal basis," Adiy said. "It is purely political, for it does not take into account the interests of citizens."

The court, in the northern Tanzanian town of Arusha, will hear the case on March 8.

The court was set up to rule on matters of the East African Community (EAC), the six-member bloc including Tanzania, as well as Burundi, Kenya, Rwanda, South Sudan and Uganda.

Some Zanzibari opposition parties have long wanted to break ties and return to the independence it briefly enjoyed in early 1964 before merging with Tanganyika.

The islands are already semi-autonomous, with their own elected parliament and president.

Elections in 2015 were repeated the following year after alleged irregularities. The opposition Civic United Front (CUF), which had promised to campaign for full autonomy of the islands, declared themselves the winners of the first poll, which was then annulled.

The long-ruling Chama Cha Mapinduzi (CCM) retained power in Zanzibar, the party of Tanzanian President John Magufuli.