UK businesses will be "severely hampered" because language skills are falling behind those in other countries, a report warns.
Ministers have been focusing effort on younger learners
The government's decision to make languages optional at GCSE in England will make the problem worse, the House of Lords European Union Committee said.
This "deep-seated ... deficiency" must be addressed, it added.
Ministers have said they are focusing more on younger children to promote a long-lasting aptitude for languages.
Their policy has been that by 2010, every child aged seven to 11 should be able to learn a language.
Last month, the government announced it would spend another £115m on providing foreign language teaching in England's schools.
However, the exams watchdog, the Qualifications and Curriculum Authority, warned recently that A-level French and German were in "chronic decline".
The peers, in a report on plans to develop the EU's Europe-wide education schemes, said: "We conclude that the United Kingdom is already falling badly behind in language-learning capability.
"This will seriously limit British ability to take part fully in and benefit from the new EU programmes."
They also said: "We are deeply disturbed by the evidence we have been given about the declining capacity for language-learning in this country."
The peers welcomed moves to improve primary school language teaching.
But their report added: "It will clearly be many years... before that has any effect on the capacity of young British adults to take advantage of EU-funded education and training schemes."
It said there were "far wider implications for the employability and cultural awareness of the coming generation and will severely hamper the country's ability to protect and promote our interests abroad and to compete successfully".
The report is called Proposed EU Integrated Action Programme for Life-long Learning.