A leading Plaid Cymru MP has suggested the security services may have carried out arson attacks on homes in north Wales in a bid to undermine the nationalist cause.

In an interview for BBC Wales' Maniffesto programme, broadcast on S4C, Elfyn Llwyd said some of the firebombings during the 1980s had been basic "petrol and match" jobs, but others had used sophisticated accelerates and timers.

"There were four categories of attacks. The first were the third party, intelligence services-led attacks, designed to undermine and blacken the nationalist movement," he said.

"Then there were the attacks committed by people who were serious about what they were doing. Number three were the copy cats and four were insurance jobs.

"It is just my opinion, but I thought this back 20 years ago that there was some third hand involvement in all of this and my belief has not changed."

Llwyd added that while the attacks "had nothing to do with us they did undermine the nationalist movement and I am sure we were set back as a result of it".

But Welsh Labour culture minister Alun Pugh said the MP had 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," he said.