One in five students who participates in an Erasmus exchange is forced to retake courses and exams after failing to be given full credit for studies abroad.

The problem of international accreditation has been highlighted in a survey of almost 9,000 students participating in the European Union exchange programme in 2010-11.

About 25 per cent of students reported receiving only partial credit for work done abroad, and 3 per cent gained no credit when returning home, according to the Erasmus Student Network.

It suggests that the loss of credits meant that many students were harming their prospects of achieving a good degree by going abroad, rather than enhancing them.

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"Lack of compatibility between courses and a lack of trust between (academic) partners is a problem," said Justyna Pisera, one of the authors of the report, Prime 2010: Problems of Recognition in Making Erasmus, published last month.

"Students are given a number of reasons why they are not receiving full credits.

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"One problem is that programmes are very different between countries. This is good because we do not want everyone to be the same; but the diversity can also be difficult, and there is sometimes a lack of trust by some professors," Ms Pisera said.

Students heading abroad on Erasmus exchanges are required to sign a learning agreement, which sets out both the home and host institution's study expectations.

But one in four agreements is not signed before the student's departure, the report says, creating uncertainty in the accreditation process.

"Learning agreements between universities are not always respected," Ms Pisera added.

"When you look at the numbers it is quite a big problem, especially for the 3 per cent of students who receive no credit at all."

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The report also highlights problems with the European Credit Transfer and Accumulation System, which allows students to carry marks achieved abroad towards their final degree.

About a quarter of universities surveyed awarded a single credit for 25 hours of study, while a quarter gave a credit for 30 hours - requiring some students to work harder.

Measurement of workloads also varied, with one country requiring students to attend 35 hours of lectures a week to gain full credit, while others included private study in the equation.

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Downgrading of marks achieved abroad was a common complaint, with different grading systems failing to translate accurately when applied to different countries.

More than 213,000 students took part in the Erasmus scheme in 2010-11, including a record 12,873 British students. About 200,000 UK students have taken part in the scheme since it was launched in 1987.

EU leaders want 20 per cent of all students to undertake a placement abroad by 2020, while the European Commission intends to increase education spending by 71 per cent from 2014 to 2020 to an average of €2.17 billion (?1.88 billion) a year.

David Hibler, Erasmus programme manager at the British Council, said students must have confidence that going abroad would not harm their grades.

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"If students believe that academic recognition is not provided consistently, then there is a risk that it will deter some from participating in the programme," he said. "This is unfortunate, not least because the evidence is that study abroad improves attainment."

jack.grove@tsleducation.com.