PORTLAND — Police and prosecutors say an Oregon Court of Appeals ruling that a drunken driving test was coerced could make it more difficult to investigate and convict suspected drunken drivers.

In a 6-4 decision this week, the court found a man's consent to a blood test was not voluntary because he agreed after a police officer read him the penalties for not complying, as required by state law.

The court also ruled the officer could have obtained a search warrant by phone.

Prosecutors and police say the ruling could change how police investigate drunken driving cases, requiring officers to get a search warrant before every breath test, blood draw or urine test.

"It could pose an enormous challenge to DUII prosecution," said Josh Marquis, Clatsop County district attorney.

Oregon Attorney General John Kroger will petition the Oregon Supreme Court to review the decision, said spokesman Tony Green.

"Given the significant on-the-ground impact of the ruling, we are working quickly on getting our appeal together," Green said.

The ruling also raises questions about the constitutionality of the state's implied consent law, which means that simply operating a vehicle implies that drivers agree to take a breath, blood or urine test if they are suspected of driving under the influence of intoxicants.

The law says refusal to take a test could be used as evidence in court, and lead to a driver's license suspension and fines.

"The whole concept of the implied consent law is that driving is a privilege, not a right," Marquis said.

Defense attorneys, meanwhile, said the appeals court ruling was a victory for individual rights.

"This is a huge case. I applaud the majority opinion for its devotion to the fundamentals of constitutional law," said Oregon City lawyer John Henry Hingson III, founding member of the National College for DUII Defense Inc., a nonprofit organization that helps train attorneys.

The ruling resulted from the 2005 arrest of Thomas Gregory Machuca after a traffic accident in Portland. Machuca was injured and taken to Oregon Health Sciences University Hospital.

A Portland police officer investigating the crash suspected alcohol was involved, and noticed a strong smell of alcohol in the emergency room when he arrived at the hospital.

The officer told Machuca he was under arrest for driving under the influence of intoxicants and reckless driving. He read the state's implied consent law to Machuca, who consented to a blood test.

Machuca argued the test should be suppressed, but a trial judge denied his motion. Machuca entered a conditional guilty plea, reserving the right to appeal.