Police investigations have been compromised by an error that led to hundreds of thousands of records being deleted from UK-wide databases, according to a letter seen by the BBC.
The National Police Chiefs' Council said 213,000 records were deleted - more than the 150,000 first reported.
This resulted in a couple of "near misses" for serious crimes when trying to identify an offender, it said.
The Home Office has said it is assessing the impact of the mistake.
Data including fingerprint, DNA, and arrest histories was wiped from the Police National Computer (PNC) - which stores and shares criminal records information across the UK - after being inadvertently flagged for deletion.
The PNC is used in police investigations and provides real-time checks on people, vehicles and crimes, as well as whether suspects are wanted for any unsolved offences.
The Home Office said the lost entries related to people who were arrested and then released without further action.
But the letter from the National Police Chiefs' Council (NPCC) says officers are aware of at least one instance where the DNA profile from a suspect in custody did not generate a match to a crime scene as expected, potentially impeding the investigation.
It says that some of the records had been marked for indefinite retention following earlier convictions for serious offences.
And it reveals that a "weeding system", developed and deployed by a Home Office PNC team, started to delete records wrongly last November.
The process was only brought to a halt at the start of this week.
The letter was sent on Friday afternoon by Deputy Chief Constable Naveed Malik of the NPCC to chief constables and police and crime commissioners.
'Risk to safety'
The deletion of the records has been blamed on a coding error.
This resulted in records that had been flagged for deletion being lost from the database before checks had been carried out to determine whether they could be lawfully held or not.
Policing minister Kit Malthouse said the problem had been identified and the process corrected so "it cannot happen again".
He said the Home Office, National Police Chiefs' Council and other law enforcement partners were working "at pace" to recover the data.
The Home Office said no records of criminal or dangerous persons had been deleted.
But Labour shadow home secretary Nick Thomas-Symonds called on Home Secretary Priti Patel to take responsibility for the error and be clear about the impact it had had.
Speaking on BBC Breakfast, he described the situation as "extraordinarily serious", adding: "Priti Patel will be responsible for criminals walking free. We're not going to be able to link suspects to crime scenes without the DNA and fingerprint evidence."
A home office source said the accusation was "scaremongering and irresponsible".
Former Cumbria Police Chief Constable Stuart Hyde told BBC Radio 4's Today programme on Friday the "very large" loss of arrest records presented a "risk to public safety".
The records are linked to police investigations that were terminated before charge (No Further Action or NFA cases) or to those where an individual had been acquitted at court.
It is not yet known how many records of each type were lost and full extent of deletions is still being investigated. A minister is expected to update the House of Commons on Monday.
It comes after about 40,000 alerts relating to European criminals were removed from the PNC following the UK's post-Brexit security deal with the EU.
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