In a major civil liberties test in the digital age, the US Supreme Court ruled Wednesday that police must obtain a warrant before searching the cell phone of a suspect.

Mobile phones deserve the same protection against "unreasonable searches and seizures" enshrined in the US constitution's Fourth Amendment as other personal property, such as homes, the top US court said.

The court, in two cases involving criminal suspects whose mobile handsets were searched by police, weighed the interest of law enforcement in finding important evidence against the civil liberties guaranteed in the constitution.

Chief Justice John Roberts, writing for the court, said that the principles of America's "Founding Fathers" still apply in the 21st century.

"Our cases have recognized that the Fourth Amendment was the founding generation's response to the reviled 'general warrants' and 'writs of assistance' of the colonial era, which allowed British officers to rummage through homes in an unrestrained search for evidence of criminal activity," Roberts wrote.

"Opposition to such searches was in fact one of the driving forces behind the Revolution itself."

Roberts said that people store vast amounts of personal data on their phones and that "the fact that technology now allows an individual to carry such information in his hand does not make the information any less worthy of the protection for which the Founders fought."

"Our answer to the question of what police must do before searching a cell phone seized incident to an arrest is accordingly simple -- get a warrant," he wrote.

The court said the few exceptions to these guidelines would be only "to protect officer safety or to preserve evidence."

- Key to digital privacy -

Roberts was joined by seven of the eight other justices, while Justice Samuel Alito wrote an opinion in support of the ruling with some reservations.

Steven Shapiro at the American Civil Liberties Union hailed the decision as important for constitutional rights.

"By recognizing that the digital revolution has transformed our expectations of privacy, today's decision is itself revolutionary and will help to protect the privacy rights of all Americans," he said in a statement.

"We have entered a new world but, as the court today recognized, our old values still apply and limit the government's ability to rummage through the intimate details of our private lives."

Nuala O'Connor at the Center for Democracy & Technology called the ruling "a tremendous victory for privacy rights," adding that "the court clearly recognizes the impact that modern technology is having on our daily lives."

Hanni Fakhoury of the Electronic Frontier Foundation said the court "recognized that the astounding amount of sensitive data stored on modern cell phones requires heightened privacy protection," and maintained that the case "should have implications for other forms of government electronic searches and surveillance."

Yousry Zakhary of the International Association of Chiefs of Police, which joined other law enforcement groups in filing briefs supporting the police officers, said however the ruling was "disappointing and will undoubtedly impact law enforcement's ability to investigate and combat crime."

The court consolidated two cases for the opinion in which lower courts reached different conclusions.

The first emerged from the arrest of David Riley, a Californian student pulled over in 2009. His car registration was found to have expired.

Police discovered loaded guns in his car and, after scouring his smartphone, found entries related to a local gang and an earlier shooting. His sentence of 15 years to life was upheld on appeal.

In the other case, Boston police in 2007 searched the call log of Brima Wurie's flip phone, which led them to an apartment where they found drugs, money and weapons.

An appeals court threw out the evidence found on Wurie's phone, saying it had been obtained unconstitutionally.

Steven Schwinn, a constitutional law professor at the John Marshall Law School in Chicago, said the ruling "breaks little new ground on Fourth Amendment analysis" but "applies a familiar framework to a relatively new technology, cell phones."

But he added that as a practical matter, police may still be able to carry out searches, albeit with some judicial oversight.

This, he said in a blog post, "should not be overly difficult, assuming that an officer can meet the requirements for a warrant: an arresting officer need only drop a seized cell phone into a Faraday (evidence) bag and obtain a warrant for a later search."