The newscaster, who chaired the Nuffield inquiry into the decline of foreign language learning in Britain, was speaking at the launch of the new National Centre for Languages.
The centre was created under the national languages strategy, which outlines the Government's intention to allow students to drop the subject at key stage 4 from 2004.
Sir Trevor told The TES: "That's not very helpful because that's exactly the age at which our competitors are redoubling their efforts. If that goes ahead we will see a great disadvantage for this country.
"The world in which we live today is so multi-ethnic and so multi-lingual that it would be a shame if we in Britain get trapped in mono-lingualism."
Sir Trevor praised the Government's pledge to offer all children the chance to learn a foreign language from the age of seven by 2010. But he added:
"We need to see more about how that is to be done because one of the problems with that is finding language teachers for primary schools.
"Charles Clarke, the Education Secretary, is well aware of the importance of this, the core points are accepted by the Government, but we must continually chivvy it to ensure that it does better."
A study to gauge language skills among primary teachers has been commissioned by the Government, which has promised money to develop teachers' skills to meet the 2010 target.
The national centre in London has been formed by merging the Centre for Information on Language Teaching with the Languages National Training Organisation. It will support learning and advise industry and the Government.
Subject Focus on languages TES Teacher magazine 24