Mr Navalny fell into a coma onboard a flight from Siberia to Moscow last year, after he was poisoned with novichok in a suspected FSB attack.
Russia’s opposition leader was transferred to a hospital in Germany for treatment on 22 August and has remained in the country ever since.
The Kremlin has repeatedly denied any involvement in the attack. This week Mr Navalny announced plans to return home on Sunday, despite efforts by Moscow to jail him.
Now Vadim Kobzev, one of Mr Navalny’s lawyers, has said his client is on a national wanted list and could be locked up as soon as he sets foot on Russian soil.
“In theory, they can detain him as soon as he arrives but initially only for 48 hours,” he said.
His client was given a suspended sentence for a 2014 conviction on charges of embezzlement and money-laundering.
Moscow claims Mr Navalny breached the terms of that charge in December when he failed to report to the Federal Penitentiary Service.
He says the state’s attempts to incarcerate him are politically motivated.
“The court can change his whole suspended sentence into a real one and give him three and a half years in jail,” Mr Kobzev added.
Leonid Volkov, an ally of Navalny, has said Mr Navalny will become the world’s most high profile political prisoner if he locked up, likening him to Nelson Mandela, and has said he would become a symbol of resistance to the Kremlin.
The Kremlin, which refers to Mr Navalny only as “the Berlin patient,” says it is up to the relevant law enforcement agencies to decide how he is treated.
Mr Navalny says his suspended sentence ended on 30 December. He also noted the European Court for Human Rights had ruled that his 2014 conviction was unlawful.
Additional reporting by Reuters.