The band, from Blackwood in South Wales, were asked to play alongside their compatriots Tom Jones and Shirley Bassey at the event, which will be shown live on Welsh television and is due to be attended by the Queen, the Duke of Edinburgh and the Prince of Wales.
But it seems the band were unmoved by the historical significance of a concert marking Welsh devolution. Yesterday, the group's spokeswoman, Terri Hall, confirmed that they turned down the invitation because they did not want to play in front of the Royals.
"They were never on the official invite list. They would not play in front of the monarchy," she said. "It is probably a political thing as well as a personal thing, but they just did not want to play in front of the Royal Family."
About 14,000 people are expected at the show, to mark the end of a day of royal engagements in Cardiff, including the Queen's official opening of the Assembly. The evening concert, being staged by BBC Wales, will also feature the Welsh pop acts Shakin' Stevens and Bonnie Tyler, as well as a recorded message from the Stereophonics.
The Manic Street Preachers have won a series of awards since their debut album Generation Terrorists in 1992. They gained more general fame after the band's guitarist, Richey Edwards, mysteriously disappeared four years ago. His abandoned car was discovered later close to a notorious suicide spot near the Severn Bridge. While his body has never been found, there have been several reported sightings of him, in the Canary Islands, Goa in Southern India and elsewhere.
Despite not taking part, the band will still have some involvement in the event. A track co-written by their lead singer, James Dean Bradfield, will be performed during the closing stages. He collaborated with playwright Patrick Jones - the brother of the band's bass player Nicky Wire - on the track "Guerrilla Tapestry", part of Jones's forthcoming poetry album Commemoration & Amnesia. It is to be performed by four actors, including Ioan Gruffudd, the star of ITV's Hornblower.