Sandra Bullock adopts baby boy and files for divorce

(AFP/GABRIEL BOUYS)

Sandra Bullock has secretly adopted a child and is divorcing her husband, the Hollywood star has revealed.

The actress has filed for divorce from Jesse James following allegations of his infidelity.

She and James, who was linked with a tattooed model, brought new baby Louis home in January after beginning the adoption process four years ago but managed to keep his arrival secret.

Bullock told People magazine's website she is now adopting Louis Bardo Bullock, a three and a half-month-old boy, who was born in New Orleans, as a single parent.

The Oscar-winner told People: "Yes, I have filed for divorce... I'm sad and I am scared."

A representative for Bullock confirmed the star had filed legal papers seeking to end her marriage, People said.

The star, 45, said she and James had decided to keep the news about the adoption to themselves until after the Oscars.

Their close friends and family, including James's children Sunny, six, Jesse Jr, 12, and Chandler, 15, all kept the news from the outside world.

The US actress, who separated from James in March, won her first Oscar this year for her role in The Blind Side.

But days later her marriage broke up because of her husband's alleged cheating.

James publicly apologised in a statement last month for causing "my wife and kids pain and embarrassment beyond comprehension", adding: "I hope one day they can find it in their hearts to forgive me."

He said that "the vast majority of the allegations reported" were "untrue and unfounded".

Bullock shot to fame in 1994 as the feisty heroine in Speed but her career flatlined in the late 1990s before she landed a supporting role in 2004's Oscar-winning Crash.

She has paid tribute to her husband at award ceremonies in the past, saying at the Golden Globes: "There's no surprise that my work got better when I met you, so thank you."

Tattooed model Michelle "Bombshell" McGee previously told US magazine In Touch that she initially made email contact with James's motorbike company looking for modelling work. Bullock was said to be making The Blind Side around this time.