Bahrain's state TV channel has accused Qatar of plotting with the kingdom's main opposition grouping to stoke anti-government unrest in 2011.
It broadcast purported recordings of telephone calls between former Qatari PM Hamad bin Jassim al-Thani and Wefaq leader Ali Salman, in which it claimed they agreed to an "escalation".
The Bahraini public prosecutor has begun an investigation into the calls.
Bahrain, Saudi Arabia, the UAE and Egypt cut links with Qatar in June.
The four countries accuse the emirate of supporting terrorist groups and of being too close to Iran - allegations the emirate has vigorously denied.
Qatar has condemned the land, sea and air restrictions put in place by its neighbours, which have forced it to import food by sea and air to meet the basic needs of its population of 2.7 million.
The state-run Bahrain news agency reported that the purported telephone conversation between Sheikh Hamad and Ali Salman "included an agreement between the two speakers on... how to deal with the situation then so as to exacerbate it and undermine Bahrain's interests and stability".
"This represents a crime of exchanging intelligence information with a foreign country to jeopardize the kingdom's national interests," it added. "The public prosecution will announce the outcome of the investigation as soon as it is over."
There was no immediate comment from the Qatari authorities or from Sheikh Hamad, who stepped down as prime minister and foreign minister in 2013.
But Wefaq said the television report was an attempt by the Bahraini government to smear it and prolong the imprisonment of its leader, who has been jailed since 2015.
LuaLua TV cited a statement from the group explaining that Sheikh Hamad had spoken to Ali Salman as part of an attempt to resolve the political crisis in Bahrain that had "the direct approval of the king".
"The call was followed by a meeting between the Qatari official and the Bahraini king at his palace in al-Safraia," it added.
"Qatar was the sponsor of the American initiative presented by the US ambassador to Bahrain, Jeffrey Feltman, as part of mediation efforts between the government and the opposition, which refutes the regime's claims of Wefaq conspiring to overthrow the regime."
Sunni-ruled Bahrain has been wracked by unrest since security forces crushed pro-democracy protests mainly led by the majority Shia community six years ago.
The king brought in troops from neighbouring Sunni-led Gulf states to end the demonstrations and restore order. The unrest left 30 civilians and five police dead.
Activists say dozens more people have been killed in clashes between protesters and security forces since then, while bomb attacks blamed on Shia militants allegedly backed by Iran have killed a number of policemen.
Ali Salman was jailed in 2015 after being convicted of inciting hatred, promoting disobedience and insulting public institutions, while Wefaq was dissolved last year for allegedly fomenting sectarian unrest.
Human rights activists said Ali Salman's trial was unfair and that the charges he faced violated his right to free expression. They also condemned the dissolution of Wefaq, saying officials had produced no evidence to support their allegations.