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Exam papers for this year's Higher English tests have had to be replaced at short notice amid concerns some questions were leaked, it has emerged.

Tens of thousands of candidates across Scotland will be sitting the exam on Thursday.

A new version of one of the two exam papers was sent out to schools and exam centres last week.

A spokesman for the exams agency, the SQA, said it was a "precautionary measure".

The BBC understands that there was a concern some of the questions could have been leaked.

The SQA spokesman said: "We identified a potential issue with one of the Higher English question papers. The appropriate adjustments were made and a new paper was distributed to centres. Our Chief Invigilators have been informed.

"We always create more content than is needed for each examination. All content, including that which is selected for the final exam, goes through the same rigorous quality checks.

"Robust procedures are in place to ensure the integrity of our qualifications - and we took swift and appropriate precautionary action to ensure candidates would not be affected and are able to sit the examination as intended."

'Security'

The SQA stressed that candidates should not be anxious about the change.

Tight security surrounds exam papers.

After they are printed, they are distributed to schools and exam centres which are expected to store them securely until shortly before each exam.

A spokesman for the EIS teaching union said: "Where there is doubt over an element of an examination paper, the sensible option is to replace that section of the paper.

"The most important consideration is that no pupil is unfairly advantaged or disadvantaged, so that the exam can deliver a fair measure of each pupil's level of attainment.

"Clearly, there will have to be a thorough investigation into the circumstances that led to this situation, but the most important issue in the short term is to ensure that any issues with the paper are rectified."

The SQA did not say just how much of the exam paper had to be changed.

However exam papers are normally carefully checked to make sure they are well set and contain an appropriate range of questions of varying degrees of difficulty.

Last year's Higher Maths paper proved harder than anticipated. The pass mark was cut to just 35%.

In a subject like Higher English, there are few - if any - answers which are simply right or wrong in a straightforward sense. Markers look for evidence of a candidate's skill and understanding in their answers.

Were a paper to prove easier or harder than it should have been, this would become apparent from the range of marks.

If this happened the pass mark and boundaries between grades would be adjusted up or down so candidates should still receive the mark they merited.

This year's exam diet begins on Wednesday.

It will be the first year when only new look Highers are on offer.

The qualifications were adjusted to fit them in with the qualifications that replaced Standard Grades two years ago.

But last year both the revamped and old-style Highers were on offer.