Scotland's exam body is facing investigation over claims it has been paying invigilators less than the living wage.

The Scottish Qualification Authority (SQA) is a living wage employer, in line with Scottish Government policy, but invigilators claim they are not being paid what they should be.

The workers, who earn fixed rates for exam sessions, claim they effectively receive £6 an hour for the longest Higher and Advanced Higher language or science exams - way below the living wage rate of £8.25 an hour stipulated for all Scottish public sector bodies.

While the SQA said it applied the same living wage policy to invigilators as it did to its staff, the Poverty Alliance, the charity that polices Scotland’s living wage accreditation scheme, confirmed it had launched an investigation after complaints were passed to it by Labour MSP Daniel Johnson.

Charity chief executive Peter Kelly said the SQA has been asked to prove that it properly monitors the fees earned by its 6,000 invigilators throughout an exam season to ensure each of them receives the living wage.

“We take this very seriously,” Mr Kelly told the Guardian. “Living wage accreditation is a voluntary scheme: employers sign up on a voluntary basis and there’s a lot of good faith involved.

"We rely a lot on the good faith of employers to do the right thing. There is clearly an issue here."

He added that the body could be stripped of its living wage employer status if it failed to show the invigilators were being paid appropriately.

The workers receive £27.15 for a morning session, which they claim can run to four hours or more, or £54.30 for a full day, which they claim can be up to eight hours.

However, those fixed fees mean they should only work three hours 20 minutes for a morning session, or six hours 40 minutes a day.

Mr Johnson said he had written to the Scottish education secretary, Angela Constance, and her successor John Swinney, and had tabled parliamentary questions at Holyrood asking for evidence the SQA was in compliance with living and minimum wage rules.

He added: "The living wage is hugely important campaign; it would be a scandal if after the importance the Scottish government has attached to it, it transpired that a key government agency was in breach of their accreditation.

"The SQA is a public body, reporting to ministers, and they have a duty to explain their pay practices."

The SQA said its fixed fees, introduced in 2010, stipulated 15 to 30 minutes of administration time and that almost three-quarters of its exams lasted under two hours, 30 minutes.

It said invigilator fees were paid at the end of each exam season and it had to intervene in very few cases of underpayment.

A statement issued by the body said: "At current rates and, on average due to variations in exam duration, SQA invigilators are paid £27.15 for each exam session they conduct, rather than hourly. Exams range from 30 minutes to three hours. Nearly three-quarters of our exams last less than 2.5 hours. This proportionate rate is above the living wage.

"Our day rates are not equivalent to an eight-hour day but based on an invigilator conducting a maximum of two sessions on any given day.

"Due to the wide variety of circumstances in individual schools across Scotland, our chief invigilators work closely with their invigilation teams to create balanced schedules.

"Clear guidance is provided to chief invigilators to ensure that invigilators are assigned to a variety of exam sessions with differing lengths of duration."