BRASILIA (Reuters) - Brazil's Supreme Court Chief Justice said on Monday that every company operating in the country is "subject to the constitution," after one of the court's judges opened an inquiry into social media platform X's owner Elon Musk.

Chief Justice Luis Roberto Barroso said in a statement that court decisions may be subject to appeal, but never deliberate non-compliance, after Musk challenged a court decision ordering X, formerly known as Twitter, to block certain accounts.

Barroso did not mention X or any other company by name.

"It is certain that any and all companies operating in Brazil are subject to the Federal Constitution, the laws and decisions of the Brazilian authorities", Barroso said in the statement.

Musk's remarks prompted Supreme Court Justice Alexandre de Moraes to open an inquiry into the billionaire on Sunday.

In an X post on Saturday evening, the billionaire accused Moraes of "brazenly and repeatedly" betraying the constitution and people of Brazil.

Moraes is investigating "digital militias" that have been accused of spreading fake news and hate messages during the government of former far-right President Jair Bolsonaro and is also leading an investigation into an alleged coup attempt by Bolsonaro.

In Monday's statement, Barroso said the "Supreme Court has acted and will keep acting to protect Brazil's institutions."

X did not immediately respond to requests for comments sent on Monday.

Brazil's Institutional Relations Minister Alexandre Padilha, the most senior member of Lula's cabinet to comment on the matter so far, said the "attack" on Moraes is "unacceptable".

"It is not just an individual justice who is being attacked; it is the Brazilian Supreme Court and all those who defend the sovereignty of our country," said Padilha on X.