Vanuatu's opposition says the prime minister Sato Kilman has no authority under Vanuatu laws to order the release of the controversial mega yacht Phocea which was seized last year.
Mr Kilman authorised its release despite a court claim against it worth more than a million US dollars.
In his letter to the yacht's owner, the Prime Minister says after his consultation with the relevant authorities, he is satisfied that the state has no legal basis to continue to detain the Phocea.
But Edward Natapei's opposition group has called on Mr Kilman to leave the matter in hands of ports and marine.
It has also questioned the prime minister's decision when local lawyer Les John Napuatu is suing Phocea, its owner and its agent in Vanuatu for producing forged documents with his signature on them.
He is seeking compensation of more than one million US dollars.
Police say the Phocea cannot sail out of Port Vila harbour because its log book is with the Transnational Crime Unit of the police.