Brexit is causing poorer children to fall further behind in learning foreign languages, says the British Council.
Parents in disadvantaged areas are telling teachers languages will be less useful after Brexit, it says.
It warns that GCSEs and A-level languages in England are seen as being hard subjects in which to get a good grade.
The government said the overall picture for language learning in England was improving.
This is a snapshot of the state of language learning in England from the organisation that promotes British culture abroad.
It warns of growing concern that GCSEs and A-levels in modern foreign languages are seen as harder than other subjects.
Some academics have recently written to Ofqual asking it to look again at the marking of language exams.
The British Council report also describes a shift in attitude, with some parents saying languages are "little use" as the UK is due to leave the European Union.
Teresa Tinsley, the report author, says secondary schools in poorer areas are reporting a very definite Brexit effect, which could lead to an even sharper decline in language learning.
She fears if languages become the preserve of the better-off, or privately educated teenagers, those from less privileged backgrounds will be even further left behind.
"If they haven't got a language, that is a closing off of opportunities for work and culturally," she said.
Schools are also reporting a reduction in activities, such as foreign exchanges, which give children the chance to experience a different culture.
'Lack of expertise'
The government's own guidance for schools says studying a foreign language from primary school onwards is a "liberation from insularity and provides an opening to other cultures".
From the age of seven to 11, pupils in England are expected to study a foreign language, either modern or ancient.
But Dr Tinsley says while some schools embrace language learning, others are struggling because of a lack of expertise or support from nearby secondary schools.
"Schools that are not achieving well are focusing on core subjects, and primary Sats tests. These are the outcomes that Ofsted will look at."
The report suggests their choice may be very limited. Only 5% of primary schools that responded to the British Council offered German.
Sankey Valley St James primary school in Warrington is one of the few to offer German as well as French.
Pupils in year 6 are encouraged to have a German pen pal and there is a lunchtime language club.
The school has a higher-than-average number of pupils from poorer families. But head teacher Deb Feltham said that made it even more important to maintain foreign language learning.
"We have great success with disadvantaged children. Through languages they learn speaking skills as well as the ability to listen."
The German ambassador in London, Peter Wittig, told the BBC the British Council report was "alarming". He said the findings were both saddening and troubling.
He described knowledge of German as a "huge asset". There was evidence that it was the most sought-after language among employers, as well as the basis for encouraging trust and understanding across borders.
"Post-Brexit the UK will - understandably and rightly - seek a new and even greater role in our globalised world. This will be facilitated if young Britons are inspired to be outward-looking and open from a very early age," he said.
"Language learning will be indispensable, and German, which is the mother tongue to more people in Europe than any other language, will remain an ideal choice.
"If we are to value and further develop our relationship with each other, we will again have to learn, in every sense, to speak each other's language."
The report comes on the day when he will be presenting an award to teachers of German.
German has disappeared from some areas in secondary schools, but French and Spanish remain more common.
A BBC languages investigation earlier this year showed the extent of the decline of French and particularly German in the last five years.
A department for education spokesperson says the government is providing "a range of support" to schools to encourage foreign language learning.