Thailand may boycott the 2018 Programme for International Student Assessment (Pisa) this August if the test organiser refuses to allow the Education Ministry to proofread the translated examination papers in advance.

A refusal to take part would be an unprecedented move. Thailand has taken part in the test since 2000.

Education Minister Teerakiat Jareonsetta confirmed he has sent a request to the organiser asking to be able to proofread.

"I am aware that examination papers should be kept secret. However, if the translation quality is poor and not done by linguistic experts, our students may not be able to understand the questions perfectly. If that's the case the test won't reflect their true abilities which is not fair on Thai students," Mr Teerakiat told the media on Thursday.

He said the wording of the examination paper is crucial because the test does not draw on memorising facts. Instead, it tests analytical skills in maths, science and reading.

"We think the Education Ministry needs to make sure the translated version is correct and comprehensible first and foremost. Critics often disregard the issue of the language barrier and have used the country's poor performance in Pisa to attack the government for failing to provide quality education," he said.

The minister added that other non-English-speaking countries have also expressed language translation concerns regarding examination papers. In addition, the ministry has also proposed the test organiser exclude the Pisa scores of Thai students from small-sized schools in rural areas when calculating the overall score of the country, as is the case in Vietnam.

"We understand that we have a gap between students in large and small schools that needs to be narrowed. But we believe that the Pisa organisers should treat Thailand and Vietnam equally in this respect," he said.

Mr Teerakiat argued that the Pisa scores of schoolchildren in Bangkok and most urban areas in Thailand were similar to their peers' in OECD countries but small schools in the country get low scores because of limited resources.

The ratio of teachers to students in Thailand is 1:20. But in rural areas, there are fewer teachers. In some cases, there is only one teacher to instruct all the students in a school.

Pisa is funded by the Organisation for Economic Cooperation and Development (OECD). Every three years it tests the proficiency in mathematics, sciences and reading of more than half a million 15-year-olds across 70 countries.

According to the latest Pisa survey in 2015, Thailand ranked 54th in maths and sciences and 57th in reading. Overall, Thailand's rank was 54th, far below other Asian countries. Singapore was the top performer in all three subjects, followed by Japan in second place. Taiwan came in fourth, China in sixth and Vietnam in eighth.

Thai students scored 415 points in maths, far lower than the international average of 490 points. In sciences, Thailand scored 421 points, well below the international average of 493. In reading, Thailand's score was 409 points, again significantly below the OECD average of 493 points.

Worryingly, Thailand's performance dropped sharply from 2012 when students scored 427 in maths, 444 in sciences and 441 in reading.