Ewan McGregor welcomes son with girlfriend Mary Elizabeth Winstead

Ewan McGregor arrives at the premiere of Disney's 'Christopher Robin' in 2018. Photo / Getty Images

Ewan McGregor is the latest celebrity to welcome a secret baby.

After keeping their pregnancy under wraps, the actor and his girlfriend Mary Elizabeth Winstead have introduced their son to the world.

McGregor's eldest daughter Clara, 25, confirmed the birth of her baby brother in an Instagram post on Sunday.

Clara, whose mother is the actor's ex-wife Eve Mavrakis, shared a snap of herself cradling the newborn as she congratulated her father, 50, and Mary, 36.

"Welcome to the world little brother congratulations to my Dad & Mary – this is the greatest gift," she wrote alongside two adorable photos.

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A post shared by Clara McGregor (@claramcgregor)

Ewan and Mary Elizabeth have been together since 2017 after they met on the set of Fargo.

The actor's ex-wife Eve Mavrakis is a production designer he met on the set of the British TV crime series Kavanagh QC. They were married in 1995, splitting in May 2017 after 22 years of marriage. Their divorce was finalised last year.

Ewan shares Clara, and daughters Esther, 19, Jamyan, 20, and Anouk, 10, with ex-wife Eve.

This isn't the first time Ewan has surprised fans with secret baby news, previously revealing in 2011 that he had adopted a girl.

He told the New York Post: "My wife and four girls – 15, two 9-year-olds and a baby of four months – often come with me if it's a holiday or my locations and their schedules allow. Sometimes we uproot them and put them in another school."

Eve Mavrakis and Ewan McGregor finalised their divorce last year. Photo / Getty Images

While eldest daughter Clara was full of joy for her dad's baby news today, she hasn't always welcomed his new partner with open arms, having called Mary Elizabeth a "piece of trash" on social media in 2018.

Clara took aim at the actress after a fan tagged her in a photo kissing Ewan at a Vanity Fair Oscars party.

In the caption, the fan had referred to Mary Elizabeth as "the most beautiful and talented woman on earth", a description that rubbed Clara the wrong way.

"Most beautiful and talented woman on earth??? Oh man y'all are delusional. The girl is a piece of trash :)," Clara commented.

Ewan and Mary Elizabeth Winstead met on the set of Fargo in 2017. Photo / Getty Images

Soon after Clara's brutal reply, another fan added of the situation: "It took TWO to undo this marriage. If a man OR woman is happy w/ their marriage no outside interference can destroy that!"

Clara shot back: "Yup it took two!! Mary & my father :)."

Speaking to The Times in August that year, Clara admitted it was an immature approach.

"It wasn't the most mature way to go about things, but I was angry and upset," she said, adding that behind-the-scenes family drama had become "a lot to deal with".

"There had been a lot building up to it and a lot to deal with – not to make excuses or anything – but, yeah, it wasn't my finest moment," she said.

Earlier this month, Clara made headlines again after attending a red carpet event with cuts on her face after being bitten by a dog.

The 25-year-old took a trip to the hospital half an hour before the premiere of The Birthday Cake, a film she co-produced which also stars her father.

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A post shared by Clara McGregor (@claramcgregor)

She took to social media to explain the nasty cuts on her face, posting along with photos from the event: "When a dog bite lands you in the ER 30 minutes before the red carpet."

Clara also posted pictures on her story of her outfit with the caption: "Make-up courtesy of dog teeth."