Niki Lauda has hit out at Nico Rosberg's decision to quit Mercedes just days after winning the world championship.

The German's decision was just as shocking to his bosses Toto Wolff and Niki Lauda as it was to the rest of the world.

"He told me it was because of stress," team chairman Lauda told Italy's La Gazzetta dello Sport.

He also told German newspaper Welt am Sonntag: "I am a notorious planner, always with an emergency plan, and even I did not expect such a thing."

Lauda admitted that he is even a little angry.

"On one hand, you have to accept if Nico wants to go," he said. "But on the other, Nico signed a two-year contract with us in August.

"What bothers me is that Nico tells us that if he had not become world champion, he would have gone on. I think he could at least have hinted at this when he agreed the contract. He says he just wanted to be champion and then stop, but the problem is that he never gave any signals that this was his concept.

"After all, 1,200 people of the F1 division at Mercedes gave him every opportunity to become world champion with a super car. And then he gave us his resignation overnight.

"This lone decision of Nico's - and I mean everyone at Brackley and with Mercedes and many people who have worked closely with him, his engineers and mechanics - has ripped a giant hole in this excellent working team."

He said he and Wolff are therefore now left to solve a "huge problem" in the weeks before Christmas.

"We are only just starting this process now, because we were really completely surprised by this whole thing. We still have nothing - no solution," said Lauda.

He admitted that the Rosberg issue has put Mercedes at a "huge disadvantage" ahead of 2017.