Bahrain's King Hamad has ratified a constitutional amendment that enables military courts to try civilians.
Officials said it would affect only those accused of acts of terrorism.
But Amnesty International warned that the measure was so vaguely worded that it could be used to try government critics, including peaceful activists.
Civilians last faced military trials during a three-month state of emergency declared in 2011 after the authorities suppressed mass pro-democracy protests.
About 300 people were convicted of political crimes during that period, after what Amnesty International said were grossly unfair tribunals characterised by a range of violations, including the admission of "confessions" obtained under torture.
The previous constitution said the jurisdiction of military courts was "confined to military offences committed by members of the Bahrain Defense Force, the National Guard and the Security Forces" unless martial law was declared.
But the new amendment says the "military judiciary shall be regulated by law, and shall delineate its jurisdiction and competencies with regards to" those forces.
After the amendment was approved by the upper house of parliament last month, Bahrain's justice minister was quoted as saying by the state news agency that military courts would "not have the authority to put on trial civilians".
"But those who engage in terrorist acts and violent crimes will be prosecuted by military courts, as their acts are considered as armed assault," he added.
Lynn Maalouf of Amnesty International said the amendment was a "disaster for the future of fair trials and justice in Bahrain".
"It is part of a broader pattern where the government uses the courts to crackdown on all forms of opposition at the expense of human rights," she warned.
In a separate development on Monday, Bahrain's highest court reportedly reduced the prison sentence handed to the leader of the main Shia Muslim opposition grouping from nine years to four.
Judicial sources and a lawyer said the Court of Cassation had cleared Sheikh Ali Salman of seeking to overthrow the political system by force but had upheld his convictions for inciting hatred and disobedience and insulting public institutions.
His now-dissolved Wefaq party helped lead the protests in 2011, which saw thousands demand greater political rights and an end to discrimination against the Shia majority by the Sunni monarchy.
The following month, King Hamad brought in troops from neighbouring Sunni-led Gulf states to restore order and crush dissent. The unrest left at least 30 civilians and five policemen dead.
Opposition activists say dozens of people have been killed in clashes between protesters and security forces since then, while bomb attacks blamed on Iran-backed militants have killed a number of policemen.
In January, the authorities executed three Shia men convicted of carrying out a 2014 bomb attack that killed three police officers.