Academics have accused the group, whose three members come from Wales, of using Welsh slang on a poster for their new album This Is My Truth - Tell Me Yours.
Peter Hughes Griffiths, who is head of Welsh teaching resources at Trinity College, Carmarthen, says Dyma'n ngwirionedd - Dwed un ti is pidgin Welsh and grammatically incorrect because it translates as "This is my truth - say yours". He says it should read Dyma'n ngwirionedd - Dwed dy un ti.
His comments highlight the ongoing battle between purists and modernists over the evolution of Welsh. One camp says the language is being dumbed down but the other welcomes "informal" Welsh.
The debate has become so intense that academics criticised the Welsh Office for failing to get its grammar right on road signs and refusing to correct them.
Mr Hughes Griffiths has been a scourge of radio and television language which he says is a bad influence on teenagers learning Welsh.
"You would have thought the group would have made the effort to make sure the poster was grammatically correct," he said.
"But as far as Welsh is concerned, it was wrong. I know with pop groups their English is often not the best but it's disappointing considering they claim to be proud of their Welsh roots.
"It's slang and like saying, 'that what was', or something similar. Of course there are differences between regional parts of Wales but these tend to be that the same words have different meanings, not a difference in grammar."
He said Welsh had gained currency with people under the age of 15 and it was important television and popular culture did not set a sloppy example: "It's disappointing that the language is being allowed to deteriorate. It's an eyesore to have something like that on a huge poster and to get it wrong. The concern is that radio and television also gets away with people speaking like this. Standards are not being kept up."
However, a spokesman for the Welsh Language Board said the poster, which hangs from a three-storey building in Cardiff, was a "brilliant gesture".
"We welcome the fact the Manic Street Preachers have produced such a massive banner in the medium of Welsh which reflects popular youth culture," he said.
"It shows how Welsh culture and language is becoming more accessible rather than being stuck with the classic idiom. If you look at the Spice Girls songs such as 'Wannabe' they are not grammatically correct but are acceptable and an important part of popular culture.
"A lot of teenagers are learning Welsh now and gestures like these make them proud to be Welsh and to be able to speak the language."
A spokeswoman for the band members, who do not speak Welsh, said they had received several letters supporting the translation, which was chosen for its colloquial appeal.
"They wanted to do something special for Wales even though they do not speak Welsh," added the spokeswoman for Hall Or Nothing, who represent the Manic Street Preachers.
"They are very proud of their Welsh heritage. They were offered a North Wales translation and one in South Wales Welsh. The decision was taken to go for the South Wales version because it was considered to be more trendy and appropriate. We did make all the right checks and have had positive feedback."Reuse content