Bahrain's state television on Wednesday accused Qatar of attempting to overthrow the government in Manama, upping the stakes in a diplomatic crisis that has seen Doha isolated in the Gulf.
Along with Saudi Arabia, the UAE and Egypt, Bahrain cut ties with Qatar in June on accusations that the emirate supports Islamist extremism and is too close to Shia Iran.
Qatar, which is Sunni-ruled like the other Gulf monarchies, has denied the allegations.
State-run Bahrain Television aired a report which claimed that neighbouring Qatar was behind anti-government protests since 2011.
It reported that, Qatar's former prime minister, Sheikh Hamad bin Jassem Al-Thani, in 2011 had contacted Ali Salman --then head of Bahrain's largest opposition group, Al-Wefaq -- and asked him to urge protesters to flood the streets and ramp up pressure on the state.
A court order, carried by state news agency BNA, said attorney general Ali al-Bouainen had launched an investigation into a series of phone calls.
In the calls, Sheikh Hamad and Salman had allegedly agreed to "work together to escalate unrest so as to harm the interests of the nation and undermine its stability, which are tantamount to the crime of communicating with a foreign state with the intention of harming national interests".
Bahrain, which has a history of disputes over Gulf islands and reefs with Qatar, is home to a Shia majority and had been rocked by Arab Spring-inspired, Shia-led protests demanding an elected government.
The Shia movement Al-Wefaq was the largest bloc in Bahrain's elected lower house of parliament until 2011, when its members resigned en masse in protest at the state's crackdown on demonstrators.
A 2016 court order dissolved Al-Wefaq for "harbouring terrorism", while its leader Salman has been behind bars since 2014.
Bahraini authorities have regularly cracked down on protests, which at times have turned violent.
Manama and Riyadh have regularly accused Iran of provoking dissent in the island kingdom, located in the Gulf between Saudi Arabia, Qatar and Iran.
Bahrain has been a key ally of the United States for decades and is home to the US Navy's Fifth Fleet.
The United States has not taken sides in the Saudi-led boycott of Qatar, another US ally which is home to the largest American air base in the region.
*This story was edited by Ahram Online