Gordon Brown has been accused of confusing the role the DNA database played in the capture of murdered Sally-Anne Bowman's killer.
Earlier, he appeared with Ms Bowman's mother as he criticised Tory plans to remove profiles of people who have not been convicted of a crime.
Human Rights group Liberty said the reason Ms Bowman's killer, Mark Dixie, was caught was not due to the database.
Labour has passed legislation for DNA profiles to be kept for six years.
The Tories want to remove profiles of people who are not convicted after three years.
Dixie was caught after he was picked up in a pub brawl. His DNA was taken and it connected him to Sally-Anne's murder in Croydon, south London, in 2005.
'Debate confused
Mr Brown paid tribute to Sally-Anne's mother Linda who he said had "suffered the most unspeakable tragedy yet still manages to be a compassionate campaigner for good".
And he added the use of DNA helps the police put the most dangerous criminals behind bars.
But Shami Chakrabarti, director of Liberty, said election fever seemed to be confusing the debate about DNA retention.
"It has been suggested that the tragic case of Sally-Anne Bowman was only solved because her murderer was 'an innocent' on the database. In fact, he was arrested for a separate violent offence and it was then that his DNA was matched to the crime scene," she said.
"We all agree that DNA taken on arrest should be checked against unsolved crimes - this is entirely different from stockpiling the DNA of innocent men, women and children for years on end".
Labour proposals to cap the time a non-convicted person's DNA records can be kept on the database to six years, were passed on Wednesday.
The European Court of Human Rights has ruled that the current position, of indefinite retention, is unlawful.
Database size
The Conservatives believe samples should be removed from the database after just three years, as in Scotland, and should only be collected from people accused of the most serious offences.
They say Dixie would have been caught earlier under their proposals. They want to make it compulsory for anyone who has committed a serious recordable offence in the past to put their DNA details onto the database, and the priority will be those convicted of violent and sexual crimes.
He had previously been convicted of three sexual offences, but his DNA details were not on the database because he committed them before it was expanded.
At present, there are about six million profiles on the national DNA database, making it the biggest in the world. Some 30,000 more are added every month. Some 975,000 of the profiles were taken from people who have never been convicted of an offence.
Home Secretary Alan Johnson has said in 2008-09, some 79 rape, murder and manslaughter cases in England and Wales were matched to DNA profiles taken from people who had been arrested but not convicted of any crime.
BBC News
