The Vienna hospital where Niki Lauda underwent a lung transplant last week says the three-time Formula One champion is breathing without assistance and his condition is improving.

The 69-year-old Austrian received a new lung last Thursday after what the Vienna General Hospital has called a "serious lung illness."

The hospital says Lauda was fully conscious 24 hours after the operation and is making "very satisfactory" progress and "all organs are functioning well."

Lauda won the Formula One title in 1975, 1977 and 1984. He suffered serious burns in a crash at the German Grand Prix in 1976 and has twice undergone kidney transplants.

Lauda was appointed as the Mercedes team's non-executive chairman in 2012 and was involved in signing Lewis Hamilton.