The CRB processed 3.9 million certificates in 2008/09
The number of people wrongly identified as criminals or mistakenly given clean records has doubled in the past year, government agency figures have shown.
There were errors in 1,570 cases in the year to 31 March - up from 680 in the the previous 12 months.
The figures from the Criminal Records Bureau (CRB), which vets people working with children, are contained in its annual report published next month.
A CRB spokesman said overall accuracy remained "extremely high" at 99.96%.
The figures apply to those wrongly branded criminals, mistakenly given a clean record or those accused of more serious offences than they actually committed.
Teachers, childminders and nurses are likely to be among the victims of the mistakes.
The agency, part of the Home Office, was set up in 2002 to check people working with youngsters for criminal convictions, cautions and reprimands.
Anybody branded a criminal by the CRB would have to go through an appeals process to clear their name.
A total of 3.9 million certificates were processed by the agency in 2008/09 - up 500,000 on the year before, its annual report shows.
The CRB said in the first 11 months of 2008/09 accuracy rates exceeded previous years, but a dip in the quality in the final month dragged the overall figure down.
A spokesman said: "As part of its continuous improvement regime, the CRB's aim is always to issue 100% of disclosures free from error.
"Disclosure quality and accuracy is heavily reliant on the information supplied by the individual and is subject to fluctuations."
Home Office Minister Lord Brett said: "In 2008 a further 18,000 unsuitable people were prevented from gaining access to children and vulnerable adults as a direct result of a CRB check, bringing the total to around 98,000 in the past five years."
Conservative shadow Home Office minister Damian Green said the CRB should be as vigilant in not hounding the innocent as it is in exposing the potentially dangerous.