Nauru has severed a long-standing arrangement to allow appeals to the high court of Australia, impacting on the rights of asylum seekers to challenge the refusal of refugee status.

The high court is the final appellate court for Nauru, under an agreement between the two countries that has been in place since 1976. It can be terminated by either government with 90 days notice.

Matthew Batsiua, a former justice minister of Nauru who is one of 19 people charged over protests outside Nauru parliament in 2015, said the Nauru solicitor general Jay Udit told his lawyers on Friday that the Nauru government had given such notice in January or December.

The Australian Department of Foreign Affairs and Trade confirmed that Australia was notified of the termination on 12 December, meaning the 90 day period concluded on 12 March.

“Nauru’s termination of the treaty is unilateral, as provided for by the terms of the treaty,” a spokeswoman said. “Australia supports Nauru’s sovereignty and its decision to terminate the treaty.”

Severing access to the high court would have a significant impact on the rights of asylum seekers held in detention on Nauru at the behest of the Australian government, the human rights lawyer George Newhouse said.

Of the 16 appeals decided by the high court under the arrangement since 1976, 11 have concerned asylum seekers disputing the refusal of their refugee status.

Of those, eight were allowed and two were dropped because refugee status had been granted in the mean time. Only one was dismissed.

“We are sending asylum seekers to an island prison where they have virtually no rights if rights of appeal to the high court are taken away,” Newhouse said.

“Everybody should be concerned if important legal rights are being taken away from vulnerable asylum seekers and Nauruan citizens in secret.”

Batsiua said he believed the decision had been made in response to the high court in October upholding an appeal by three of the so-called Nauru 19 against a bid by the Nauru government to increase the length of their prison sentences.

The three men were re-sentenced to longer prison terms by the Nauru supreme court on Thursday, and again stated their intention to appeal to the high court.

“This step by the Nauruan government to cut the link with the high court of Australia before an alternative is in place could leave not just us, but all Nauruan citizens and the asylum seekers here without any avenue of appeal from decisions made in the supreme court,” Batsiua said.

“One of the avenues that we have been using is appealing to the high court of Australia for certain decisions that have gone against us, and we have had some success with that. So we believe the government is reacting to those situations and they are trying to shut off those avenues.”

The Nauru president, Baron Waqa, spoke about the proposed change last month, telling parliament that establishing a court of final appeal on Nauruan soil was an “affirmation of Nauru’s sovereignty, independence, and maturity as a nation”.

“Severance of ties to Australia’s highest court is a logical step towards full nationhood and an expression of confidence in Nauru’s ability to determine its own destiny,” he said.

Waqa said the change would be made as part of the constitutional amendment bill 2018, but Batsiua said that was not due to be debated by parliament until late April.

Newhouse called on the foreign affairs minister, Julie Bishop, to address the issue, calling it a “constitutional crisis”.

The Melbourne University law professor Jeremy Gans wrote in an analysis last month that the number of appeals from Nauru had jumped from five between 1976 and 2016 to 13 in 2017.

About 1,000 asylum seekers, including children, remain detained on Nauru in conditions the United Nations has warned are “devastating” to their mental health.