A civil rights group has released a new smartphone 'stealth' app which allows users to record video of police encounters without the officer's knowledge.

Called the 'Police Tape', the app makes the phone act as if it is off when recording video displaying a blank screen and when audio is being recorded, the app automatically minimises and disappears.

Designed by the New Jersey branch of the American Civil Liberties Union (ACLU) to be used as a deterent against wayward and abusive police officers, the state follows New York with the creation of a police surveillance app.

'Police often videotape civilians and civilians have a constitutionally protected right to videotape police,' said Alexander Shalom of the ACLU to the Star Ledger.

'When people know they're being watched, they tend to behave well.'

The app, which will be first available for Android users and then iPhones towards the end of the month has a simple instructional menu panel which offers legal advice, and then the option to record video or audio.

'This app provides an essential tool for police accountability,' said Deborah Jacobs, executive diretor of the ACLU of New Jersey.

Last month the New York Civil Liverties Union launched a similar smartphone app called 'Stop and Frisk Watch' to counteract what it feels are controversial practices by the New York Police Department.

The signature feature of the 'Police Tape' app is the instant uploading of any recorded content automatically to the ACLU server.

This is to combat any deletions by police officers who have been videoed or recorded saying something or doing something that they wouldn't have wanted committed to tape.

However, police groups have greeted the new app cautiously.

'Guys are basically told, conduct yourself as if you're always being recorded, that's the safest way,' said Chris Tyminski, president of a police association in Essex County in New Jersey.

'It's unfair when groups like the ACLU judge a life or death split second decision that a cop makes, when they have days and roundtables to discuss what a cop should have done in those three seconds.'

Countering the claim that the promotion of the 'Police Tape' will lead to dangerous attitudes towards law enforcement, Shalom was clear.

'Police officers who break the rules, who don't behave, are the exception not the rule,' said Shalom.

'It's only the minority of officers who are flouting the rules who should be concerned about the app.'

Some though, have offered a more pointed opinion on videoing police officers, stealthily or otherwise.

'I also hope that if a police officer is attempting to stop an individual on the street, that person is not suddenly trying to pull a phone from his pocket in an attempt to film a police encounter,' said James Stewart, president of the Newark Fraternal Order of Police.