Foreign students may have been unfairly deported from the UK after being falsely accused of cheating in English language tests, a report has warned.
The government withdrew 30,000 visas from non-EU citizens after it emerged students were having other people sit their tests for them.
The National Audit Office concluded cheating had been "large scale", but innocent people may have been deported.
Thousands accused of cheating had since won leave to remain, it also said.
The Home Office said the NAO's report highlighted "the scale and organised nature of the abuse", adding that 25 people had received criminal convictions for their role in the scandal.
In 2014, BBC Panorama broadcast footage showing organised cheating in two English language test centres run by third parties for the non-profit organisation Educational Testing Service (ETS).
The service used voice recognition technology to try to find out who had cheated by having someone else sit their test.
ETS classified 97% of UK tests taken between 2011 and 2014 as suspicious, 58% as invalid, and 39% as questionable.
In the wake of the findings, the Home Office shut down colleges, excluded students and cancelled visas.
The head of the NAO, Sir Amyas Morse, said the Home Office "should have taken an equally vigorous approach to protecting those who did not cheat but who were still caught up in the process, however small a proportion they might be".
"This did not happen," he added.
The watchdog said it could not estimate accurately how many innocent people may have been wrongly identified as cheats.
As of March, 11,000 people who had taken the English tests had left the country after an accusation was made against them, the NAO said.
One student accused in 2014, who did not want to use his real name, told the BBC's Today programme the Home Office tried to deport him in 2017 and kept him in a detention centre for seven days.
"All this actually made my life really miserable, I cannot even finish my studies," he said.
At the beginning of May he launched an appeal to clear his name. He is waiting to hear the result.
Last month, a woman who came to the UK from Bangladesh in 2010 told the Victoria Derbyshire programme she also had at one stage been detained after being accused of cheating.
Fatema Chowdhury, who finished her law degree at the University of London in 2014, denied the allegation and said no evidence of her alleged cheating had been presented to her.
She is not currently being threatened with removal from the UK, but while remaining in the country cannot work or use the NHS for free.
She said her "dreams and hopes" had disappeared, and she was "desperate" to speak to someone at the Home Office to "prove my innocence".