About 70 of the Britons whose convictions for crimes abroad were left off police records may have reoffended in the UK, the BBC has learned.
Conservatives accused Mr Reid of being "out of touch"
There may be others who have new names or have gone abroad again, the National Association of Probation Officers said.
Meanwhile, the Home Office played down reports about career criminal Dale Miller, who carried out a killing after returning to the UK.
It denied he would have been under supervision, had records been updated.
The government has come under repeated fire this week after it emerged that 27,500 files, including 540 for serious crimes, had not been entered on the police national computer.
It is now thought that about 70 of those 540 serious offenders have been convicted of a further offence since returning to the UK.
The head of the National Association of Probation Officers, Harry Fletcher, said the figure, based on feedback from officers, was particularly low as half of all offenders are usually expected to reoffend within two years.
"It seems very low - but that may be because many of them have disappeared, changed their names or even gone abroad again," he added.
The Observer reports that among those to reoffend was British gangster Dale Miller, convicted of violent offences abroad during the 1990s.
Dale Miller was sentenced to 16 years for manslaughter
The newspaper claims his convictions were not on record until shortly before he was sentenced to 16 years in jail for manslaughter in Newcastle in 2002.
But a Home Office spokesman said it was incorrect that he would have been supervised by probation officers if his records had been stored electronically earlier.
"Dale Miller has no implications for today's figures which are aimed at identifying those people who may have slipped through the net, still be at liberty and working in positions with children and other vulnerable adults," he added.
Liberal Democrat home affairs spokesman Nick Clegg said the case of Dale Miller was unlikely to be an isolated one and called for an independent, rather than an internal, inquiry.
"The Home Secretary is not in a position to act as both judge and jury on the government's lamentable record in the Home Office," he said.
Home Secretary John Reid has said he wants to reassure the public he was working to ensure their protection.
Earlier, his department released figures showing that its failure to update police records had not led to any violent or sexual offenders being cleared to work with children.
Five of the 540 most serious offenders had sought checks with the CRB for employment purposes which did not show up their crimes, the Home Office said.
A spokeswoman confirmed that four of these had been convicted for drugs offences in Europe. The other had been convicted of assisting illegal entry to a country.
She confirmed that two were applying for jobs as sports coaches, two had sought employment as carers and one as a foster carer.
A further nine cases had been found where individuals with similar details to those on the list had undergone CRB checks.
Efforts were continuing to establish whether they were actually the same people, the spokeswoman added.
Shadow home secretary David Davis called Mr Reid "out of touch" and said the announcement would not quell people's fears.