Attempts to derail The Prince of Wales investiture are revealed in a new book

But the device, created from a tube to free the Welsh from the "English yoke," barely caused a ripple when it was tested, leading the Free Wales Army (FWA) and its self-proclaimed leader, Cayo Evans back to the drawing board.

The anecdote, told by journalist Lyn Ebenezer, features in the book by author Arwel Vittle .

It explores the explosive start to King Charles III's royal career and how, according to nationalists, the "archaic and oppressive tradition has been a blight on the nation for centuries," since Edward I deposed the last native Prince of Wales, Llywelyn ap Gruffudd in 1282.

It also details the bizarre plots to disrupt the 1969 ceremony at Caernarfon Castle, which also saw four other bombs planted by the militant group, MAC (Mudiad Amddiffyn Cymru).

In the book, Lyn Ebenezer recalls travelling to a remote area with Evans in the run up to the royal proceedings where the 'bomb' made by a Llangollen man, was being tested.

He took cover behind a stone wall.

"I saw the smoke rise from the bomb in the wall, and then after a few seconds came a noise: ‘Pffft.’ A cloud of smoke rose up but no stone was dislodged. Sheep were still quietly grazing and none raised their heads."

The FWA first appeared in public at a 1965 protest against the construction of the Llyn Celyn reservoir near Bala.

The consequence of the stunts and exploits of the FWA diverted attention from the the MAC, masterminded by John Jenkins who was radicalised by the drowning of the Tryweryn Valley above Bala in Gwynedd.

The Welsh nationalist and British Army soldier was jailed for 10 years for organising explosions in a campaign of sabotage against the investiture.

One device exploded unexpectedly killing two members of the MAC in Abergele.

One exploded in a police constable's garden in Caernarfon during a 21-gun salute, the second was found by a 10-year-old Buckinghamshire boy playing football while on holiday, who lost part of his leg when it exploded.

Author Vittle, who runs a translation company, said it was "interesting" to hear the first hand accounts of the activists and extremists at the heart of the protest movement.

"With Charles becoming King and his coronation yet to take place, I wanted to write a popular history book which was a good read as well as informing."

Charles and the Welsh Revolt is published by Y Lolfa and available to buy online and in book stores.