Tens of thousands of murderers, rapists and violent criminals could escape prosecution following a "bizarre" legal ruling.
The ruling, made by a district judge at Salford Magistrates' Court and backed by the High Court, means an end to the practice of releasing people on bail and calling them back for further questioning later - a common practice in most major inquiries.
Police forces can no longer put anyone out on bail for more than 96 hours without either being in a position to charge or release them.
After the four days is up, officers can no longer question suspects and can only re-arrest them if they have new evidence, the ruling says.
Police chiefs have been left baffled by the "bizarre" ruling and both the Association of Chief Police Officers (Acpo) and the Crown Prosecution Service (CPS) are currently considering the ramifications for forces across England and Wales.
Home Secretary Theresa May said: "I think this is a matter of great concern.
"We're working with Acpo at the moment and looking at a number of possibilities as to how we can advise the police on this issue.
"There may be an opportunity to appeal this decision
"We are also looking at whether or not it's necessary to introduce legislation in order to deal with this issue.
"We are conscious of the concerns this judgment has brought in terms of operational policing."
Sir Norman Bettison, chief constable of West Yorkshire Police, said: "This means unless this is overturned police can no longer put anyone out on bail for more than 96 hours without either being in a position to charge or release.
"It's on the verge of a disaster now because the question being asked by my custody sergeants is, 'What do we do, boss?'
"I cannot countenance turning people away from the charge office and telling them all bets are off and they are free to go."
He went on: "We are running round like headless chickens this morning wondering what this means to the nature of justice.
"My holding position with my officers is that I can't believe this is what was envisioned.
"We are awaiting advice from the CPS.
"The early indications are that until this matter is appealed or new legislation is passed the issue of putting people on bail for further questioning when they answer their bail is pretty much a dead duck.
"We are waiting, as the rest of the world is, for the best advice from the best legal minds.
"It's a mess."
Sir Norman added he was telling his officers to continue working as they have always worked, ensuring no suspect spends more than a maximum of 96 hours actually in custody, until further guidance was issued.
About 4,260 suspects are currently on bail from his force alone - which represents about 5% of the police service - meaning about 85,200 people are on bail at any one time, he said.
The district judge at Salford Magistrates' Court ruled that the detention clock continues to run while the suspect is on bail from the police station.
Paul Hookway, a murder suspect, was first arrested at 12.40pm on November 7 last year.
A superintendent granted permission for him to be detained for up to 36 hours for questioning, but he was released on bail after about 28 hours.
Five months later, on April 5, police applied to the courts to extend the period of detention from 36 hours to the maximum allowed of 96 hours.
But the district judge refused the application, saying that the 96 hours had expired months ago.
Greater Manchester Police applied to the High Court for a judicial review of the case, but Mr Justice McCombe upheld the district judge's decision on May 19 and refused leave to appeal.
The force is now seeking leave to appeal to the Supreme Court.
Assistant Chief Constable Andy Adams, Acpo's lead spokesman on custody issues, said: "As it stands, this ruling has a profound impact on how Pace (Police and Criminal Evidence Act) has been interpreted and used during the last 25 years.
"This issue needs clarification so that all those involved in the administration of justice can be clear about the impact and consequences."
He went on: "We are working in partnership with colleagues across the criminal justice system, particularly the Crown Prosecution Service, and have commissioned a QC to advise on the ruling and its impact on policing.
"We are working with the Home Office to seek to reduce any immediate impact before the expedited hearing at the Supreme Court.
"The decision in this case has the potential for a wide-reaching impact and Acpo has significant concerns as to the effect it will have on policing."
James Welch, legal director for the civil rights group Liberty, said: "Being out on bail pending investigation is not the equivalent of being detained.
"Limits on the time that suspects can be held in police custody are necessary but there are good reasons why the police should be allowed to bail suspects for more than 96 hours.
"If this decision cannot be appealed legislation should be introduced to clarify the law.
"This would also be an opportunity to introduce safeguards into the system and to extend the regime to include terror suspects and end the scandal of punishment without trial."
Mark Leech, editor of the national prisoners' newspaper ConVerse, said: "There is an urgent need for reform in this area.
"Police practice to date has been there is no limit to the period someone can be placed on police bail but that cannot be right - the current system doesn't serve victims, it doesn't serve suspects, and it certainly doesn't serve the public.
"A balance has to be struck between giving police a reasonable period to complete their inquiries on one hand, and the right of a suspect to be freed of the shackles of police bail on the other - bail which can involve curfews, daily reporting to a police station and surrendering of a passport - all before a single charge has been laid against them."
Chief Constable Jim Baker-McCardle, the Acpo lead on the Hookway ruling, said it has a "profound impact on how the police have investigated crime under a legal framework interpreted and used during the last 25 years".
"Unless overturned, the indications are its effect is that police can no longer put anyone out on bail for more than 96 hours without either being in a position to charge or release," he said.
"We are working in partnership with colleagues across the criminal justice system, including the Crown Prosecution Service and Home Office, to determine a sensible way forward and seek to reduce any immediate impact. Chief officers have significant concerns as to the effect it will have on policing."
Shadow home secretary Yvette Cooper said the issue was "a matter of grave concern for police across the country".
"They have suddenly been told they may not be able to recall suspects who are out on police bail for further questioning, an ID parade or other investigation unless there is new evidence in place," she said.
"Because it seems this has immediate effect, it will disrupt vital ongoing investigations and hugely hamper the police in their job.
"Police officers I have spoken to are deeply alarmed at the implications for criminal cases they are working on right now."
She went on: "The Home Office have known about this judgment for six weeks.
"The Home Secretary needs to explain urgently what her legal advice says and what she will do to sort this problem and make sure thousands of ongoing investigations aren't lost.
"If the only answer is emergency legislation to temporarily restore the old arrangements until further work can be completed, then we stand ready to help the Home Secretary get this through.
"But the public and police need an urgent answer so important cases aren't put at risk."