Jørn Utzon, the Danish architect who designed the Sydney Opera House, has died at the age of 90. Mr Utzon died from a heart attack in his sleep at his home in Copenhagen.

"He had not been doing well these past few days, since Thursday. He had been undergoing a series of operations recently," Mr Utzon's son, Kim, told the Associated Press.

The first design for the Sydney Opera House was created by Utzon in 1957. Mr Utzon left Australia and returned to Denmark in 1966 due to disputes with the NSW government over cost blowouts and design differences. The building was finished 1973. Utzon never returned to Australia to see his building completed.

Relations between New South Wales and Utzon improved in the late 1990s and in 1999 the Sydney Opera House Trust asked for Mr Utzon to advise on changing the building's interior to match his original design.

Mr Utzon was awarded the Pritzker prize for architecture in 2003. The selectors said the Opera House was one of the most iconic buildings of the 20th century saying that Utzon's design "proves that the marvellous and seemingly impossible in architecture can be achieved".

Kim Williams, chairman of the Opera House Trust, said the building would dim the lights on its sails on Sunday as a sign of respect. Mr Williams said he had met Mr Utzon at his home in Denmark recently, where they shared emotions and tears.

"We spoke for 40 minutes and when we left, he embraced me and he whispered in my ear 'no tears'," he said.

"He hasn't been able to come back himself, but through his son he has been keeping up with all the developments here at the Opera House, really on a daily basis."

Premier of NSW Nathan Rees said NSW was "deeply saddened" by the death of Mr Utzon, who gave Australia a "masterpiece" which was now UNESCO World-Heritage listed.

Australian Prime Minister Kevin Rudd said Mr Utzon had left an impressive legacy.

"Joern Utzon was a visionary architect whose legacy includes one of the world's most spectacular and inspiring buildings, the Sydney Opera House," said the Prime Minister in a statement.

"Standing proudly on the edge of Sydney Harbour, the Opera House is one of the most internationally recognised symbols of our nation."