President-elect Donald Trump once again sparked controversy with an early morning social media post.

On Twitter, Trump suggested Americans who burn the flag should be jailed or even lose their citizenship.

For the past 27 years, burning the flag has been legal.

"What the president-elect said this morning flies directly in the face of well-established constitutional law," attorney Dan Recht said.

Recht has practiced constitutional law for nearly four decades.

"In 1989, the U.S. Supreme Court dealt with this very issue," he said.

The case was Texas v. Johnson, where the court had to decide if the symbolic act of flag burning is a First Amendment right.

"And the Supreme Court decided in fact flag burning as a type of speech, as a type of expression was protected by the First Amendment," Recht said.

But it was a close decision, 5-4 with conservative justice Antonin Scalia tipping the decision in favor of protecting flag burners.

The decision made state laws prohibiting people from desecrating the flag unconstitutional.

After Tuesday's tweet from President-Elect Trump, the reaction on social media included some who support punishment, like Travis Ragan.

"So glad someone is standing up for our country. Burning our flag in no way should it be legal and the people who do it should be punished for doing so," Ragan posted on the 9NEWS Facebook page.

"I have never understood how burning the American flag under protest is a freedom of speech. So I will never grasp the idea that it is a protected constitutional right. I find it wrong and disgraceful," said Brittney Salas on the 9NEWS Facebook page.

The ACLU of Colorado said even though it is protected, having the president-elect question the legality of flag burning creates fear.

"Any time that a leader tries to suppress protest and dissent, nothing could be more unpatriotic than that. It is a bedrock of our democracy that we can protest, that we can criticize our government and our leaders," said Nathan Woodliff-Stanley, executive director for the ACLU of Colorado.

In the 1989 vote, Justice Scalia sided with the majority in the case, but he was clear he did not like flag burning. Scalia's death in February created an opening on the Supreme Court which Trump says he will fill with someone like the late justice.