Nascar champion Kyle Busch died after severe pneumonia progressed into sepsis, his family has said in a statement to the Athletic magazine.

A medical evaluation showed the illness resulted "in rapid and overwhelming associated complications", the statement said.

Busch died suddenly aged 41 on Thursday after what his family had described as a "severe illness".

Nascar described Busch as a "rare talent, one who comes along once in a generation". He won two Cup Series titles and 63 races.

Busch had been in the middle of his 22nd season in Nascar's top division at the time of his death.

He had been expected to compete in an upcoming race this weekend, the Coca-Cola 600 at the Charlotte Motor Speedway in North Carolina.

He is survived by his parents, his wife and two young children.

Nascar CEO Steve O'Donnell told a news conference that the races would continue, as Busch would have been upset were they not to go ahead.

"So we're going to honour his memory," O'Donnell said.

Busch's number and his truck were on display ahead of the event, according to local media.

"Kyle Busch lived just about every chapter of what you could do in Nascar," said O'Donnell, who described the late driver as fierce, competitive and "always good for a great quote".

O'Connell also praised Busch's charity work, citing the foundation he started with his wife to support awareness and advancement for IVF and infertility treatments.

In addition to his winning record, Busch also built a larger than life persona in the racing world and was known by the nickname "Rowdy".

Other tributes poured in following the announcement of Busch's sudden death.

"Absolutely cannot comprehend this news," former teammate Denny Hamlin wrote on social media. "We just need to think of his family during this time. We love you KB."

Over the course of his 20-year career, Busch won 234 races in Nascar's top three series, more than any driver in history, according to ESPN.

Sepsis is a serious reaction to an infection which can be life-threatening.

According to the UK's NHS, its exact cause is not known – but it is thought to occur when the immune system overreacts and starts to damage the body's organs.

It can be triggered by any type of infection, but bacterial infections are the most common.

It usually develops quickly over a few hours and requires immediate intervention. It can turn into septic shock if untreated, which can be fatal.

Common symptoms in adults include confusion or slurred speech, chills, difficulty breathing, extreme muscle or body pain, and discoloured skin. Symptoms can be harder to spot in babies and young children.

According to the CDC, at least 1.7m adults and more than 18,000 children in the US develop sepsis each year.