The Cornish Republican Army (CRA) says it carried out the attack last month as part of a renewed campaign to prevent the “ethnic cleansing of the people of Kernow”.
Formerly known as the Cornish National Liberation Army (CNLA), the group has been dormant for over a decade and was believed to have been disbanded.
But in an official blog announcing the name change, the group made a series of chilling threats - including that it has a would-be suicide bomber in its ranks.
The group promised a prolonged campaign against second homeowners who have priced locals out of the county.
“We have ceased activities against Stein and Oliver, but our activities against second and expensive English owned homes will continue,” the statement said.
“We are in no hurry to conduct actions and so these will happen over a period of time.”
The statement - which has now been removed from the website - even claimed the groups has a female martyr who is willing to die for the cause.
“Our organisation has grown and we now have one member who is prepared to pay the ultimate price in the battle for Kernow.
“She is prepared to sacrifice herself although we shall not ask for this lightly - only as a last measure.”
The posh eatery Rick Stein Porthleven was badly damaged by fire in the early hours of Monday 12 June.
A lean-to bin store which housed three gas cylinders was set alight and firefighters who extinguished the blaze said it was just 15 minutes from destroying the restaurant.
Investigations into the cause of the blaze are ongoing and arson has not yet been ruled out.
But now the alleged terror group has come out of hiding to claim it carried out the attack along with two other recent fires in Truro and Penryn.
It claims to have set off a “practice device” at the former Redruth Brewery site and had removed dozens of “red blood flags” - believed to be a reference to the St George Cross.
The CRA statement said the group has 30 volunteers who are operating in Active Service Units (ASU) to bring about Cornish independence.
It claimed that ‘activities’ against Stein and Jamie Oliver - who also has a restaurant in Cornwall - have now ceased.
But the group vowed to target the second homes of wealthy English people and the authorities of what it calls the “English Imperial System” or EIS.
The statement said: “We are also responsible for the damage to Stein’s Restaurant.
“The fact that the EIS police arrested someone in Coventry is a ‘red herring’.
“We have removed literally scores of Blood Cross flags and destroyed them.
“We have also issued warnings to owners of holiday businesses flying the blood cross flag.”
It continued: “We also intend to target those authorities including the EIS police who victimise Cornish people. What have we to fear or lose - nothing.
“We note that others are damaging the English Tudor Signs and we support this action. Letters sent to the EIS are wasted efforts.
“The English have never allowed freedom to any peoples without a fight and although we understand those who believe that freedom can be won through democracy or political means, we do not accept this.
“What has really been won this past 50 years? - nothing other than an ethnic cleansing of the people of Kernow.”
The incident in Porthleven follows years of bitterness between Stein and the local community where he has taken over many businesses and angered locals with his “arrogance.”
He spent a large part of his fortune on a string of fish and chip shops and restaurants in the port of Padstow on the north coast of Cornwall.
The success of the empire in the town led to it being nicknamed ‘Padstein’.
Three years ago Stein expanded further by taking over an existing restaurant in the tiny port of Porthleven on the south coast.
His move into the village angered many locals - including several fisherman - after he snubbed them despite promoting fresh food from local waters on his menu.
The CRA claims to have new funding from other Celtic groups and says it changed its name because of copycats who had prompted “many arrests”.
And it likened its crusade to the IRA, whose former members are now part of government in Northern Ireland.
“Whilst we greatly admire and respect the efforts of Cornish Nationals who seek change through moderate means, we note that previous members of the IRA are now in power in the North of Ireland showing that there is a place for military action.
“Therefore we may use direct means to underline the efforts of moderate Cornish Nationals.”
The Cornish National Liberation Army first emerged in the early 2000s and claimed a number of arson attacks in the county.
Previously known as ‘An Gof’, the group claimed to have funding from Welsh activists responsible for burning holiday homes in Wales in the 1980s.
In 2012 a letter emerged claiming the group had a 12-year-old member - but nothing has been heard since and it was believed the organisation and ceased existence.
And despite the chilling threats, a disclaimer above the statement appears to contradict itself by claiming the group supports “legal political activity only”.
It reads: “While we understand and have sympathy for those Cornish people who are frustrated and angry about the English Imperialist occupation, oppression and ethnic cleansing of Kernow, foreign ownership of second homes and the uncontrolled influx of incomers into our homeland - we seek to Free Cornwall for the native indigenous Celtic Cornish folk by legal political activity only.”