The findings contradict recent claims by Schools Minister Diana Johnson that the Government's strategy to get more young children speaking another language was working.
They also cast doubt over the viability of one of the Government's key education pledges which entitles all seven-to-eleven-year-olds to learn a modern language by 2010.
Ms McLachlan said: "Because the quality and quantity of children's exposure to primary language teaching varies so much, it's often impossible to teach a class in secondary school with such a wide range of skills.
"Perhaps even worse, some secondary school children will have to endure lessons which they have been already taught at primary.
"The fact that primary languages are not inspected by Ofsted and that students are still entitled to opt out at the end of Key Stage 3 tells it all.
"Languages - either at primary or secondary level - are just not a priority for the Government despite what they say. If we follow this course, we are doomed to retain our global reputation as the least proficient and most unenthusiastic linguists in the world.
"It's fantasy to expect that somehow this inadequate primary system will magically instil an enthusiasm for languages in secondary school: it's just not happening."
The study, published in the Language Learning Journal, also revealed inadequate training arrangements and the difficulties primary schools have in receiving funding for language teaching.
She added: "It's little wonder primary teachers have little appetite for making time on the school curriculum for this subject - especially as it doesn't contribute to league table places."
