NIKI Lauda has passed away, aged 70.

The Austrian F1 legend died on Monday, May 20.

What was is cause of death?

His cause of death has not been revealed – although he had a lung transplant eight months ago.

The driver was severely burned in a near-fatal crash in 1976 and incredibly came back to the sport, winning his second world championship a year later.

But his severe fireball injuries and burns – contracted 42 years ago – left him battling health problems for the rest of his life

He had two kidney transplants, with one donated by his brother Florian in 1997 and another from his wife Birgit in 2005.

The Austrian racer also underwent a lung transplant in August last year after falling ill with flu-like symptoms while on holiday in Ibiza.

He had the procedure in Vienna, with medics revealing he had been suffering from “severe lung disease”.

Since his lung transplant, the 70-year-old’s road to recovery has been slowed by setbacks including a flu infection.

Earlier this month, he was transferred to a private clinic so he could have dialysis treatment for his kidney problems.

Speaking 10 days ago, his brother Florian said: “Niki is making progress but he is still in Switzerland”.

What is known is that Lauda died peacefully, with his family.

How many children did he have?

Lauda had two sons with first wife Marlene Knaus, who he married in 1976 and divorced in 1991 – Mathias who is a racing driver too and Lukas who acts as Mathias's manager.

He also had a son, Christoph, through an extra-marital relationship.

Lauda married Birgit Wetzinger in 2008, and in September 2009 Birgit gave birth to twins – a boy and a girl.