The German government has accused Vietnamese intelligence services of involvement in what it called the kidnapping in Berlin of a former Vietnamese oil executive.
Trinh Xuan Thanh, 51, disappeared in July last year after he was accused of mismanagement at a subsidiary of national oil and gas giant PetroVietnam, resulting in losses of some 150 million dollars (£113 million).
Vietnamese police issued an arrest warrant in September.
This week, Vietnamese authorities said he turned himself in to police in his homeland on Monday.
German authorities, however, believe that he was kidnapped in Berlin on July 23.
They say that he had sought asylum in Germany – an application that had not yet been processed – and that Vietnamese authorities had sought his extradition.
“There is no longer any serious doubt about the participation of the Vietnamese intelligence services and the embassy … in the kidnapping of a Vietnamese citizen in Berlin,” German foreign ministry spokesman Martin Schaefer told reporters.
The kidnapping, he added, “is an unprecedented and flagrant violation of German and international law” and “has the potential to negatively influence relations in a massive way”.
Vietnam’s ambassador was summoned to the foreign ministry on Tuesday and was told that Germany demanded that Mr Thanh be returned so that the asylum and extradition proceedings could be conducted properly.
Mr Schaefer said that German and Vietnamese officials had met on the sidelines of the Group of 20 summit on July 7-8 to discuss Hanoi’s wish to have Mr Thanh extradited.
Germany is declaring the intelligence attache at Vietnam’s embassy persona non grata and demanding that he leave within 48 hours, Mr Schaefer said.
“We reserve the right to draw further consequences if necessary at a political, economic and development policy level,” he added.
Mr Thanh was chairman of PetroVietnam Construction Joint Stock Corporation until 2013, when he was appointed to several senior government positions, including vice chairman of Hau Giang province in the southern Mekong Delta.
He was elected to the National Assembly in May 2016, but was dismissed from the communist-dominated legislature before its first session the following month.
The ruling Communist Party and government have stepped up their anti-corruption drive over the past few years.