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Exam board Edexcel has launched an investigation into how part of an A-level maths paper was leaked online.

Blacked-out images of two questions were shared on social media on Thursday afternoon.

Pearson, which runs Edexcel, said the images were circulated "in a very limited way" shortly before Friday's Maths Paper 3 exam.

It reassured students no-one would be advantaged or disadvantaged and they would not have to re-sit the paper.

The questions were first posted on Twitter in a now deleted post; the account holder urged students to get in touch, offering the whole paper for £70.

It is the third year in a row that A-level maths questions from an Edexcel paper were revealed online ahead of the exam.

Student, 17, Reading

"To come out [of the exam] and find that there were screenshots of a group chat, where there were pictures of the whole paper - and not just blacked-out images - was extremely disheartening.

"If the grade boundaries go up because everyone that cheated performed much better, all of us that actually put in the work for the past two years will not receive the grades we need.

"We also don't understand how Edexcel are going to find out who cheated and who didn't, as pictures circulated quickly online - so people may have received it without even asking for it.

"Stressing over maths A-level has been an awful addition to an already stressful exam period, and has affected the quality of revision for my exams in the following days.

"I really hope it's third time lucky for Edexcel and they can sort this leak out, because students have lost all faith in this exam board."

The leaks in 2017 and 2018 were investigated by the police and evidence was passed to the Crown Prosecution Service for consideration over whether criminal charges should be brought.

Pearson said it had taken action to identify the source of this week's breach and was "determined to identify the individual involved".

After visiting a small group of exam centres, the company said it had identified one centre deemed to be "in serious breach of correct practice".

Earlier this year, Pearson said it would be trialling a scheme where microchips were placed in exam packs to track the date, time and location of the bundles.

Hundreds of students have signed a petition calling for Edexcel to take the leak into consideration when they set grade boundaries.

One student who sat the exam told BBC Radio 4's Today programme he was "frustrated" to hear about the leak.

"After I came out of the exam, I was speaking to someone and they said 'those leaked questions on Twitter came up on the paper' - and I was like, 'what leaked questions?'".

"I'm not frustrated because I feel like I absolutely failed the paper, it wasn't that kind of frustration," he added.

"It's more the frustration that I've spent two years studying maths A-level only to realise that some students, through some sort of opportunity, could have not learnt anything for two years - and just spent £70 and got an A-level."

Geoff Barton, the general secretary of the Association of School and College Leaders, says schools and exam boards face a number of new challenges.

"We are in a new age [of] technology," said Mr Barton.

"Social media, the ability of young people to have phones in exams... all of that has changed the territory," he told the BBC.

"We place far too much emphasis on the sheer number of exams we expect young people to do," he added.