Cate Blanchett has given birth to third son Ignatius Martin on Sunday in a Sydney hospital

Cate and her playwright husband Andrew Upton added another child to their brood on Sunday, a son named Ignatius Martin Upton, said a spokesman for the Sydney Theatre Company .

The couple already have two sons: Dashiell John, aged six, and Roman Robert, aged three.

A spokesman for the Sydney Theatre Company, of which the couple are co-directors said: "The baby was born yesterday. Both are doing well".

In November, when she confirmed her pregnancy, Blanchett, 38, was asked whether she was carrying a boy, but she said: "You know more than me. It's early days yet. It's due in April."

The 'Elizabeth: The Golden Age' and 'I'm Not There' star's new son weighed in at 3.6kg, or just under eight pounds, according to sources.

Cate missed out on a second Oscar at this year's ceremony, losing out to Tilda Swinton and Marion Cotillard in two female acting categories.

She is shortly to star alongside Harrison Ford as Agent Irina Spalko in the fourth installment of the Indiana Jones series, titled "Indiana Jones and the Kingdom of the Crystal Skull."

Meanwhile, Blanchett has been picked by Prime Minister Kevin Rudd to head a think-tank on the future of arts in her home country this weekend as past of a summit on challenges facing Australia, leading 100 of the country's best minds.

Actress Cate, seen here with her playwright husband Andrew Upton, and her new baby boy are both said to be 'doing well'