English examiners have complained about the use of slang expressions such as 'gonna', 'ain't' and 'shouda' in GCSEs this year.
Markers for the exam board Edexcel said "almost unforgivable" basic errors were made by apparently bright pupils.
In its annual report on its English GCSE papers, Edexcel said there were "many concerns about elementary errors in the work of apparently able pupils". Another complaint was the failure to use a capital letter for 'I'.
The verb forms "shouda", "gonna", "ain't" and "wanna", appeared with surprising regularity, the examiners said.
Candidates should not use street language or the style used in text messages, they said.
Examiners had more praise for the standard of A-level English Literature.
But they were concerned at the inability of some candidates to spell Shakespeare, in a paper called "Shakespeare in context".