The German government on Monday announced the start of a voluntary recall of 630,000 vehicles to fix emissions irregularities uncovered in the wake of Volkswagen's "Dieselgate" scandal.

The federal transport authority KBA said in a statement it would begin the recall with refits for 10,500 Porsche Macan 4x4s, one of the luxury automaker's best sellers.

A German inquiry in April found that the diesel-powered Macans were among a number of car brands that deactivate exhaust treatment features below a certain outside temperature.

Without processing, the exhaust does not meet European emissions standards for pollutants including harmful nitrogen oxides.

Berlin's transport ministry accepted the carmakers' defence that the "temperature window" on the Macan and other vehicles was meant to prevent damage to the motor.

That means unlike many vehicles from Porsche parent Volkswagen, which admitted in September 2015 to using manipulating software to fool regulatory emissions tests, Porsche and other manufacturers including Daimler and General Motors subsidiary Opel are subject only to a voluntary recall.

"All the affected German manufacturers have committed themselves in writing" to the voluntary retrofits, the KBA said in a statement.

The new narrower temperature window corresponds to "the actually necessary extent" for protecting the engine, it went on.

The German transport ministry has also ordered a compulsory refit of some 2.4 million Volkswagen vehicles on the country's roads.

Recalls began in January, but have been subject to delays as VW and the KBA wrangle over the details of the retrofits to each model.

Across Europe, the VW group -- which owns 12 brands ranging from lower-end Skoda and Seat to luxury Audi and Bentley -- plans to recall around 8.5 million vehicles over the emissions cheating scandal.