Schools in England will have to ensure at least half their pupils study a foreign language until they are 16.
Fewer teenagers have been taking GCSEs in modern languages
The target was announced by schools minister Jacqui Smith, who said schools should encourage language study.
Last year the government removed the compulsion for all secondary school pupils to study a modern language up to GCSE or the equivalent level.
A recent poll said only a quarter of England's secondary schools made pupils over 14 study a language.
The new guidelines come in from September 2006.
There has been an outcry over a decline in the number of pupils studying modern languages from the age of 14. Until that time, pupils have to study a language.
UK entries in French this summer were down 14.4% from 2004 to 272,140, while those for German exams decreased by 13.7% to 105,288.
Schools minister Jacqui Smith said: "Speaking a second language enhances understanding of our own culture and opens the door to others. It enables us to communicate with other people on their terms, rather than always on ours.
"And it offers enhanced job prospects and the opportunity to travel.
"We are committed to encouraging young people to study languages, while recognising that they should be offered flexibility in what they study.
"Schools must articulate the arguments in favour of language study and to do all that they can to encourage take-up so that pupils don't miss out."
When it removed the compulsion to study languages up to GCSE level, the government brought in plans to boost modern language learning in primary schools.
The target is for all seven to 11-year-olds "to be entitled to" learn at least one foreign language by 2010.
The government's action has been praised by campaigners. Isabella Moore, director of Cilt, the National Language Centre, said: "We are delighted that the government has taken a strong stance in relation to pupil drop-out from languages.
"Schools now have a clear definition of entitlement and a very strong message about the level of take-up they are expected to deliver.
"With this in mind, we hope they will not only safeguard existing provision but also look to developing vital new courses which are motivating to pupils and relevant to their future needs."
Languages are not compulsory at GCSE in Wales (or Scotland at the equivalent level).
In Northern Ireland, schools have to teach languages to age 16. However, they can already "opt out" of this requirement, and around half have so far.
From 2008, GCSE languages will be non-compulsory across the board, as in the rest of the UK.
Shadow minister for schools, Nick Gibb, said the Conservatives supported the government's action.
"In this global age it has never been more important for this country to have its young people learn foreign languages, particularly Spanish, and the Chinese and Arabic languages," he said.
"It is, however, deeply regrettable that the government removed the compulsion to study a foreign language from the age of 14, a mistake they are now being forced to remedy with these new measures."
Liberal Democrat education spokesman Edward Davey said: "This government's answer to every problem is to set targets.
"The government needs to reform the over-regulated curriculum and recruit more qualified specialist language teachers.
"Students on the continent are becoming fluent in three or four languages. We cannot hope to be competitive in the world if we are allowing the next generation of Britons to miss out on these vital skills."