Thompson gives the girls at her old school an English lesson

(GETTY)

She has ingratiated herself with the nation's youth through her role as the somewhat unhinged Professor Sybil Trelawney in the Harry Potter films, but now actress Emma Thompson has had something of a swipe at the Muggle population of her own alma mater.

"I went to give a talk at my old school and the girls were all doing 'likes' and 'innits?' and 'it aint's', which drives me insane," she told the Radio Times. "I told them, 'Don't do it because it makes you sound stupid and you're not stupid.' There is the necessity to have two languages – one you use with your mates and the other that you need in any official capacity. Or you're going to sound like a knob."

Her comments may come as a surprise to the well-to-do parents of pupils at the Camden School for Girls, a comprehensive in North London long known as "the Left's finishing school" with a list of glittering alumni. Sarah Brown attended, as did her former business partner Julia Hobsbawm, the daughter of the historian Eric Hobsbawm. Arabella Weir, Fiona Millar the journalist and education campaigner and former Spice Girl Geri Halliwell are also former pupils.

At a charity function for parents last year, the former poet laureate Andrew Motion and Tony Blair's pollster Lord Gould became involved in a bidding war for tickets to Garsington Opera, which had been donated by the conductor Steuart Bedford (Gould eventually won, paying £550).

Ms Thompson's comments about the necessity of speaking two different types of English are not without foundation. According to a recent study, half of teenagers are unable to tell the difference between standard English grammar and colloquial language, prompting concern that the use of social networking websites and the prevalence of mobile phone text messaging is undermining children's literacy skills.

In a study by Cambridge Assessment, one of the country's largest examination boards, fewer than six in 10 pupils correctly identified "off of", "she was stood" and "this man showed us" as ungrammatical. The study surveyed more than 2,000 teenagers in 26 English secondary schools.

Ian McNeilly, from the National Association for the Teaching of English, said: "For a lot of people their daily use of English is in new media, where non-standard grammatical constructions are more acceptable. That's inevitably going to lead to an increased lack of awareness of more standard constructions."

It is easy to see why the English language is currently at the forefront of Ms Thompson's mind - she is currently writing a new version of the classic musical My Fair Lady, in which a poor flower girl is transformed into a high society lady by a phonetics professor, Henry Higgins.

British actress Carey Mulligan will take the role of Eliza Doolittle – played by Audrey Hepburn in 1964 – but she will doubtless suffer none of the linguistic afflictions of the Camden schoolgirls. She learned English at the International School in Dusseldorf, Germany, before relocating to the prestigious, independent Woldingham School in Surrey.

Don't be a hater, Emma. Chillax, blood

Editorial, Viewspaper, page 2