Volkswagen has set a deadline at the end of November for its whistleblower programme designed to encourage workers to disclose information about the car maker's two emissions scandals in a move to speed up investigations. 

Europe's largest carmaker has been making slow progress in finding out who had knowledge of the rigging of diesel emissions tests two months after the manipulations became public in the US. 

Last week it also admitted to cheating on carbon dioxide emissions certifications. 

Under the whistleblower programme, approved by VW's top management, workers who get in touch with internal investigators no later than November 30 will be exempt from dismissals and damage claims.

This is according to a letter from VW brand chief Herbert Diess to staff seen by Reuters. 

"We are counting on your cooperation and knowledge as our company's employees to get to the bottom of the diesel and CO2 issue," Diess was quoted as saying in the document. "In this process, every single day counts."

His comments confirmed an earlier report by Sueddeutsche Zeitung jointly with German broadcasters NDR and WDR.

VW has said it hired advisory firm Deloitte and US law firm Jones Day to investigate under what circumstances the company installed software into diesel cars that changed engine settings to reduce emissions whenever the vehicle was put through tests. 

VW is eager to show federal prosecutors in Detroit and Washington that it is fully co-operating with the criminal investigation into the company's admitted use of "defeat devices" in 482,000 US diesel vehicles. 

US Attorney Preet Bharara in New York in September cited General Motors' co-operation as a factor in the government's decision to impose a $900m fine for the delayed ignition switch recall linked to 124 deaths and 275 injuries. 

By contrast, Bharara's office imposed a fine of $1.2 billion in March 2014 for Toyota's delayed recall over sudden acceleration problems which were linked to about a half-dozen deaths. 

GM's internal investigation was led by Anton Valukas, a former US attorney in Chicago, whose firm interviewed 230 witnesses over 70 days and reviewed 41 million pages of documents.

Bharara at a news conference cited GM's "fairly extraordinary" co-operation and the internal report in ending the government's investigation into GM's conduct on an expedited basis. 

Under the three-year Justice Department settlement, GM was required to establish a toll-free phone number to allow whistleblower employees to anonymously disclose problems to Schwartz. 

By contrast, Bharara said Toyota failed to co-operate thoroughly or quickly enough, which he said was one reason for the higher fine and longer investigation.