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

Under the whistleblower programme, approved by VW's top management, workers who get in touch with internal investigators no later than 30 November will be exempt from dismissals and damage claims, according to a letter from VW brand chief Herbert Diess to staff seen by the Reuters news agency.

"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 the Sueddeutsche Zeitung newspaper jointly with German broadcasters NDR and WDR.

The news agency said VW was eager to show federal prosecutors in Detroit and Washington that it is fully cooperating 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' cooperation as a factor in the government's decision to impose a US$900m fine for the delayed ignition switch recall linked to 124 deaths and 275 injuries. By contrast, his office imposed a fine of $1.2bn in March 2014 for Toyota Motor's delayed recall over sudden acceleration problems which were linked to only 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" cooperation and the internal report in ending the government's investigation into GM’s conduct on an expedited basis, Reuters noted. GM lawyers, Bharara said, gave US prosecutors real-time updates on the investigation – "often revealing to the office what witnesses had said – even before GM management was filled in … That cooperation is the reason we are here after only 18 months in a complicated case – rather than four years or more."

As part of the settlement, GM agreed to oversight by an independent monitor who will ensure it makes safety reforms. Last month, the government named former federal prosecutor Bart Schwartz to oversee the agreement.

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, Reuters said.

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

Meanwhile, according to Reuters, the portfolio manager at Union Investment, which holds 0.5% of Volkswagen's preference shares, on Thursday urged the company to name new top leadership.

"We would prefer people from outside VW to lead the management and supervisory boards," Ingo Speich said in a statement made available to Reuters. "What matters most now is to regain trust of the capital markets; this cannot be done with the current leadership."

He said the company's current leaders are tainted by their long involvement with Volkswagen. "VW has had its crises over the past 10 years but never taken drastic measures. This is their chance to finally take the right action."

A source at VW told the news agency the executive and supervisory boards initially sought to have the whistleblower programme run through the end of the year but, encouraged by recent positive feedback, decided to set the more ambitious end-November deadline.

Whistleblower programme were successfully employed years ago by German engineering group Siemens and VW's truck-making subsidiary MAN to help unveil incidents of corruption amid ongoing bribery probes, the report added.