So, how far can the needs of crime-fighters outweigh claims to personal privacy? Technology now allows someone to bug a conversation from across the streets, which can just as easily pick up someone's intimate and innocent secrets as help catch a criminal.
The use of intrusive surveillance by law enforcement agencies will inevitably increase as they concentrate on targeting the criminal rather than the crime, raising parallel concerns about safeguards.
Legislation is due in the new year to allow MI5 agents to join the fight against organised crime alongside their traditional beat countering terrorism, espionage and subversion.
The key questions will be how their new role balances secrecy with accountability and how their skills in covert intelligence-gathering will fit into the demands of an open-court process.
Ironically, MI5's use of intrusive surveillance techniques is more strictly regulated than the police's, following the European Commission on Human Rights' commendation of the unjustified surveillance of leading civil rights campaigners Harriet Harman and Patricia Hewitt, which led to the 1989 Security Services Act. The workings of the complaints process, though, remain secret.
John Wadham, director of the civil rights organisation Liberty, is opposed to any MI5 involvement in police matters."In the case of the police, the issue is not whether it is right for them to use these covert techniques; the issue is whether there are adequate controls," he says.
"At the moment the police can follow you and bug you and, because we have no Bill of Rights or legal right to privacy, these tactics are completely unregulated except by internal guidelines."
Liberty launched its campaign for a Bill of Rights last week to mark Sunday's International Human Rights Day with its focus on safeguards against the power of the state.
One case due to be heard by the House of Lords in March illustrates the need to find a balance between the prevention of crime and the rights of the individual.
Sultan Khan was jailed in 1993 for three years for his involvement in the importation of heroin. He initially pleaded not guilty at Sheffield crown court. However, he changed his plea after the trial judge ruled admissible the only evidence against him - a tape of a conversation picked up by a bugging device planted by South Yorkshire police at the house of another man whom they were watching.
Jeremy Brooke, legal executive with Graysons, the Sheffield law firm representing Khan, admits: "Most people take the view that if he is guilty, then surely the evidence should be allowed. But there are important principles involved about the lack of regulation governing the use of surveillance techniques and the lack of remedies if the intrusion is unwarranted."
The trial judge and the Court of Appeal, which dismissed Khan's appeal against conviction in 1994, both took the view, based on R v Sang, that regardless of how evidence has been gathered, it is admissible if it is relevant.
The appeal to the Lords will look at the admissibility argument and will then consider whether the judge could have used his discretion under Section 78 of the Police and Criminal Evidence Act to exclude the evidence if he felt it was prejudicial to the fairness of the proceedings.
Mr Brooke said the police worked on the basis that, under English law, they could do what they liked, providing there was not a law prohibiting it. Since the use of covert techniques was not regulated by statue, there was nothing to stop them using them.
"This changes the onus completely from Article 8 of the European Convention on Human Rights, which states that any interference with privacy must be in accordance with the law and that requires clear written law," he says.
Khan, who is legally aided, was given leave to appeal to the Lords because the appeal judges felt there was an important point of law to be clarified. However, other bugging cases, where the Interception of Communications Act 1985 did not apply, have had appeals refused. Mr Brooke says that if they are not successful, they will seek to refer the case to the European Court of Human Rights.
Lord Lester of Herne Hill QC, who proposed a Human Rights Bill in the last session of Parliament, said the involvement of MI5 in countering organised crime was a difficult area. Both the European Court and the UK courts tended to tilt the balance firmly in favour of protecting the security of the state.
He favours statutory regulations similar to the German system, which has three levels of oversight.
"A Bill of Rights would only be a long stop in these cases because it is hard to expect judges to strike the balance when they do not really know enough about the way the service operates," he says.
Bill Taylor, chairman of the Association of Chief Police Officers' crime committee and Commissioner of the City of London Police, says the police are in favour of a statutory framework to regulate use of intrusive surveillance to avoid any anomaly with MI5. Legislation is now waiting parliamentary time.
"Surveillance, intelligence-gathering and use of information have been bread and water to us since the inception of the police," he says. "Their use as evidence is for the court to decide, based on case law, which is why some cases will always been seen to be pushing at the boundaries."
Organisations such as Liberty and Justice provide important "rubbing points" in the system. "If we didn't have them challenging us, there would be the risk of law enforcement officers going too fast, too quickly," Mr Taylor says. "But we also have a responsibility to point out things we believe are wrong with the criminal justice system and say certain approaches are needed for us to be effective - are you prepared to give them to us?"
Graham Martin, the Crown Prosecution Service's head of prosecutions, says decisions on whether or not to use covert techniques are operational matters for the police. Crown prosecutors, however, can advise on the legal consequences or any potential difficulties that could arise if the case came to trial.
The involvement of MI5 agents in investigations will inevitably be raised by defence counsel, raising potential conflicts over disclosure. Senior police officers say they have dropped about 100 cases against major criminals in the past 18 months rather than reveal sensitive information. The law is currently being amended to require the defence to disclose its case earlier.
Mr Martin says: "The more sophisticated the undercover techniques, the more difficulties arise because the more reluctant the police are for those techniques to be generally known.
"However, I can't see a way of making these problems go away without creating a serious risk of injustice. I suspect in the end we have to accept that the price of trying to make sure innocent people don't go to prison is that some guilty people escape conviction."