The Supreme Court ruled on Wednesday that lawyers for people thinking of pleading guilty to a crime must advise their clients who are not citizens about the possibility that they will be deported.

Likening deportation to the punishments of banishment and exile, Justice John Paul Stevens, writing for five justices, said the Constitution guaranteed competent legal advice on at least some collateral consequences of guilty pleas.

“It is our responsibility under the Constitution to ensure that no criminal defendant — whether a citizen or not — is left to the mercies of incompetent counsel,” Justice Stevens wrote.

The vote was 7 to 2, though two justices in the majority would have required only that criminal defense lawyers not say anything false and tell their clients to consult an immigration lawyer if they had questions.

The case involved Jose Padilla, a native of Honduras who has lived in the United States for 40 years, served in the Vietnam War and is a legal permanent resident. Mr. Padilla, a commercial truck driver, was arrested in 2001 after the authorities in Kentucky found more than 1,000 pounds of marijuana in his truck.

Mr. Padilla pleaded guilty to marijuana trafficking, a felony, and received a five-year sentence. He later said he had agreed to the plea based on his lawyer’s incorrect advice that it would not affect his immigration status. In fact, the plea made it all but certain that Mr. Padilla would be deported once he served his time.

The question in the case, Padilla v. Kentucky, No. 08-651, was whether bad legal advice about a collateral consequence of a guilty plea could amount to ineffective assistance of counsel under the Sixth Amendment.