The programme to introduce languages into primary schools has resulted in "amateurish" teaching with scant resources and potentially bad pronunciation, teachers will tell the ATL annual conference next week.
Helen Brook, who studied French at school, will describe teaching Spanish at her Cambridgeshire primary as "terrifying" and potentially insulting to properly trained languages teachers.
In a speech to the conference, she will claim that the Government should to re-evaluate the statutory teaching of languages in primaries, because more funding, training and curriculum time need to be made available.
She will say that many schools rely on brave members of staff to stay "two pages ahead" in the text book, in order to make language provision available.
She told The TES: "I think it's really commendable that children should be learning a modern language at primary school, but I don't think the programme has been well thought out.
"There is a lack of funding, time and trained teachers. There needs to be more professionalism. I volunteered to teach Spanish, but I have the equivalent in Spanish to what Manuel from Fawlty Towers has in English.
"I haven't a clue if I'm teaching the pupils anything wrongly, especially the pronunciation. I've ended up really enjoying teaching it, but I was terrified when I started, and it is really amateurish.
"I also wonder about whether our secondary school colleagues find it insulting to them. They are properly trained and I'm here allegedly teaching these children."
She said she was also concerned that secondary teachers might end up having to "unteach" mistakes.
Ms Brook's comments echo the conclusions of several recent academic studies into the impact of the primary languages drive, which was first announced in 2002.
Last September, a report from Manchester University described the delivery of the initiative as "catastrophically diverse", while a study by Cambridge University found the scheme had had very little impact at secondary level.
Last July, the National Federation for Educational Research concluded that nearly a quarter of primaries were unprepared for compulsory languages teaching, which will come into force in Year 3 of primary schools in 2011. By 2014 languages will be compulsory throughout key stage 2.
A spokesperson for the Department for Children, Schools and Families said #163;7 million had been spent training 5,500 primary teachers with a languages specialism since 2002.
A further 900 training places are available for 201011.