A Plaid Cymru MP believes UK security services were involved in some arson attacks blamed on Welsh extremists.
Meibion Glyndwr was linked to more than 200 fire bombings of property
It is 25 years since the start of 12 years of fire-bombings, attributed to a shadowy group known as Meibion Glyndwr.
Elfyn Llwyd has suggested the security services could have been involved, with the intention of discrediting the nationalist vote.
But Welsh Culture Minister Alun Pugh said Mr Llwyd had "been watching too many episodes of the X Files".
In March this year, North Wales Police reopened the case, saying materials kept during their investigations would be examined to find whether it would yield DNA evidence.
Meibion Glyndwr - which means "sons of Glyndwr" - began burning property in December 1979 in protest at homes in rural Wales being sold as holiday cottages to people from England.
The group was linked to most of the 220 or so fire-bombing incidents stretching from the Llyn Peninsula to Pembrokeshire. The campaign continued until the early 1990s.
Police were accused in some quarters of targeting anyone who was a nationalist. Although one man, Sion Aubrey Roberts, was convicted in 1993 of sending letter bombs in the post, the arson cases remain unsolved.
Detectives hope DNA evidence may still lead to arrests in the case
As a solicitor, Elfyn Llwyd represented Welsh singer Bryn Fôn when he was arrested on suspicion of being involved in the arson campaign. Fôn was released without charge .
But now, as MP for Merionnydd Nant Conwy and Plaid Cymru's Parliamentary Leader, Mr Llwyd has argued that some of the terror attacks may have had the involvement of the security services and not Meibion Glyndwr.
He believes that elements of the British security services may have carried out renegade actions in order to discredit Plaid Cymru and the nationalist vote ahead of elections.
The claim is made in an interview for BBC Wales' Maniffesto programme to be shown on S4C on Sunday.
Theory is questioned
Mr Llwyd said that the sophistication of many of the devices used in the attacks compared to the crude nature of many others, suggests a degree of professionalism which could only have come from individuals who knew exactly what they were doing.
He said: "What I'm saying is that the role that they took wasn't the appropriate one, i.e. like an agent provocateur and perhaps interfering and creating a situation where it looked like it was the nationalists that were responsible."
Culture and Welsh language Minister Alun Pugh dismissed Mr Llwyd's theory.
The Clwyd West AM said on Friday: "Mr Llwyd has been watching too many episodes of the X Files.
"The reason why people don't vote nationalist is not because of some exotic conspiracy involving the secret services, it's because of their barmy policies and daft leaders. Case closed."
The programme also heard from Lord Roberts of Conwy, who was a Welsh Office minister at the time. He denied that the security services played any improper role.
"There was a rumour that, perhaps, whoever did the foul deed was also familiar with the police force, but I don't believe it," he added.
Mr Llwyd's theory has also been questioned by Plaid Cymru's former President, Dafydd Wigley.
He accepted that the fires damaged Plaid Cymru's public image but believed that the security services had their hands full at the time with the IRA and animal rights activists.
Maniffesto can be seen on S4C on Sunday, 12 December, at 1200 GMT.