The Malaysian government has dismissed the graft allegations linked to a $1.1-billion submarine purchase in 2002, when Najib was defence minister, saying it is an opposition-backed attempt to smear his image.
"William Bourdon was deported late Friday to Paris on Malaysia Airlines carrier," E. Nalini, programme manager of human rights group the Voice of the Malaysian People (Suaram), told AFP.
Bourdon, Suaram's lawyer in France, was arrested early Friday by immigration officials when his plane arrived at Kuala Lumpur International Airport on a domestic flight from nothern Penang island.
At the request of Suaram, French judicial officials opened an investigation in March 2010 into the sale of the two Scorpene submarines, which were made by French shipbuilder giant DCNS.
Malaysian officials said Bourdon was deported because he had violated his social visit visa by giving a speech at a dinner in Penang organised by Suaram on Thursday. The lawyer had visited Malaysia previously.
Suaran condemned the authorities' action, claiming his arrest was unlawful.
"The arrest was unlawful and the government has been unfair to deport Bourdon," Nalini said.
Suaram alleges DCNS paid a commission of €114 million ($160 million) to a company called Perimekar, which is linked to Abdul Razak Baginda, an associate of Najib's.
Abdul Razak has been acquitted of charges of abetting the 2006 murder of his mistress, Mongolian interpreter Altantuya Shaariibuu, which the opposition has also been trying to link to Najib.
But Najib has denied any link to that case. The Malaysian government has also maintained that the submarine deal, brokered when Najib was defence minister, was free of graft and that Perimekar had not improperly benefited.