Former Brazil World Cup winner Ronaldinho, who was arrested for possessing a fake Paraguayan passport, must remain in jail in the South American country after a judge on Tuesday refused a request for him to be released into house arrest.

The former Barcelona forward and his brother and business manager Roberto Assis were arrested last Friday after they were caught carrying the false Paraguayan documents. Brazilians do not need a passport to enter Paraguay so it was unclear why Ronaldinho had the fake documents in his possession.

The brothers will remain in a maximum security prison on the outskirts of Asuncion for as long as six months, or until the investigation is complete, judge Gustavo Amarilla said.

"Ronaldinho's freedom could lead to an obstruction [of justice] or an escape," Amarilla told reporters. "His presence in the country is necessary."

The brothers offered a $770,000 property as a guarantee but prosecutor Marcelo Pecchi argued they were a flight risk.

"The investigation is only beginning, we are looking into other conducts that may be related," Pecchi told reporters.

"If these people leave right now they will not be able to be brought to face trial because Brazil does not extradite its own citizens."

Although he last played professionally in 2015, Ronaldinho is still hugely popular with global football fans and advertisers and was invited to Paraguay by a local casino owner.