Girls are more than twice as likely as boys to pass a GCSE in a modern foreign language, a report suggests.
Just 38% of boys in England took a foreign language at GCSE in 2018, compared with about 50% of girls, a report for the British Council says.
Using statistical modelling, the Education Policy Institute study found when factors like background and ability were accounted for, boys were 2.17 times less likely to succeed.
But some schools are bucking the trend.
Researchers used a set of characteristics to model the likelihood of different types of pupils achieving a pass in a language GCSE, finding different results for different groups.
In most areas of education, the biggest achievement gap is between disadvantaged children and their more advantaged peers.
In languages, however, a pupil's gender has the biggest effect on the likelihood of whether they will succeed.
Boys, overall, had an odds ratio of 0.46, where one means success, while disadvantaged pupils had a ratio of 0.57.
'Small percentage pass'
The report found that a girl from a poorer background was more likely to outperform a boy from a more affluent background in modern languages.
The report notes that many schools enter large numbers of boys for modern language GCSEs for accountability reasons, but only a small percentage of them pass.
Researchers highlighted a more inclusive, all-abilities approach as the reason behind the increase in boys studying and succeeding at languages in some successful schools.
Overall entries in languages have seen a significant decline in recent years.
The English Baccalaureate, a wrap-around qualification which requires pupils to sit GCSEs in English, maths, two sciences, a humanities subject and a modern foreign language, aimed to address this decline.
Report author Bobbie Mills said: "The government has set an ambitious target of 75% of pupils studying the EBacc by 2022.
"If it intends to make any progress towards this goal, it must urgently clarify how it intends to address the huge gender gap in languages.
"There is no evidence that current initiatives to improve foreign language entries will narrow this divide".
The report urged the exams watchdog, Ofqual, to address the difficulty of languages at GCSE.
It has already said that French and German will be marked more favourably from 2020, after a review of grading.
It should look at whether changes are required in other modern foreign languages, the report said.
The Department for Education said the introduction of the EBacc had helped halt the decline in take-up of GCSE languages, with 47% of pupils taking a language in 2019, up from 40% in 2010, and the proportion of boys remaining "broadly stable".
"We are committed to ensuring more pupils are studying languages, which is why it is now compulsory in the national curriculum for all children between Years 3 and 9," an official said.