DNA samples taken from thousands of former prisoners could be destroyed if police lose a judicial review later.
An ex-prisoner, referred to in court as R, argues that a police force's request to collect his DNA threatens arrest and infringes his human rights.
Under Operation Nutmeg, DNA samples have been collected from prisoners who pre-date routine collection.
Judges will decide if the force's approach was unlawful and if DNA collected in such a way can be kept.
The police force at the centre of the case - which also cannot be named for legal reasons - was trying to collect the sample as part of Operation Nutmeg, a push across England and Wales to collect genetic material from people jailed for serious crimes before 1994.
After that date, people convicted of serious crimes had DNA swabs routinely taken to add to the national database.
Liable to arrest
The aim of the operation is to see if there is any match to unsolved crimes with DNA from the former prisoners.
By July of this year, 6,204 samples had been taken under the scheme, with 111 being matched to crime scenes.
R - who was jailed for manslaughter in the 1980s but after his release was only in trouble for a lesser, non-violent offence - argues that he has turned his life around since 2000.
In a statement read to the court, he said: "I have changed my life over the past 13 years and have earned the right not to come under suspicion."
R was contacted by police in March this year. An officer hand-delivered a pro forma letter which told him that because he had a previous conviction for a serious offence he was being asked to give the officer a DNA sample.
The letter went on to say that if he chose not to, he would be required to attend a police station within seven days and if he failed to do that he could be liable to arrest.
The letter was signed by the chief constable of the force.
The BBC understands such pro forma requests are made by many other forces, as part of Operation Nutmeg.
The force involved in the case sent out 391 such letters, and collected 389 DNA swabs there and then.
R's lawyers told a hearing in July that the letter he received breached national guidelines because the threat of arrest made it a requirement to give the sample, not a request.
They also allege Operation Nutmeg breached section 8 of the Human Rights Act - the right to respect for private life.
They added that there was no reason to suspect R had committed any other crimes and to ask for his DNA was "pure speculation" in case there was a match on the files.
The judges hearing the review - Lord Justice Pitchford and Mr Justice Hickinbottom - indicated in the earlier hearing that they were inclined to agree that the letter breached the guidelines.
That could mean all the Operation Nutmeg DNA samples collected using similar methods may have to be destroyed, and criminal cases based on such evidence could collapse.
If they agree that the complainant's human rights were breached, it could affect the way DNA samples are collected in the future.