A spokesperson for the actor told The Sun: "I can confirm that Hugh Bonneville and Lulu Williams have separated."
HELLO! has contacted a representative for comment.
The couple tied the knot in 1998 and welcomed their son, Felix, shortly after. Meanwhile, on their 20th wedding anniversary, the former couple travelled to the Graceland Wedding Chapel in Las Vegas to renew their vows.
While Hugh, 59, tends to keep his family out of the spotlight, he did share a rare glimpse inside his life as a father during a candid chat with online magazine, Parents.
"I think the absolute key one is don't force your child to do the things you failed to do and wanted to do, like playing the piano or something," he said.
"Another great piece of advice that someone once said to me when my little boy was arriving was, 'Hug him close, and let him fly.'"
News of Hugh and Lulu's split comes after the actor stepped out in London for his co-star Michelle Dockery's fairytale nuptials.
Hugh, who starred as Lord Grantham in the popular period drama,looked every inch the 'proud father' as he mingled with wedding guests on Michelle's big day.
For the special occasion, Hugh donned an off-white suit which he paired with a bright blue silk tie. He was pictured with the likes of Jim Carter who starred as his trusty butler Carson on the hit ITV show.
Hugh and Jim weren't the only members of the Downton Abbey cast to witness Michelle and her beau Jasper Waller-Bridge exchange vows at the altar.
Laura Carmichael, who played Michelle's on-screen sister Lady Edith Crawley, attended as a bridesmaid. The actress, 37, looked beautiful in a sea-green gown which perfectly complemented Michelle's gorgeous Emilia Wickstead white satin wedding dress.
Also in attendance was Joanne Froggatt who looked nothing short of sophisticated in a teal-hued maxi dresscrafted by Ghost complete with a billowing A-line skirt, capped sleeves and myriad buttons running down the front.
The actress, who starred as Anna Bates in the hugely popular period drama, spruced up her look with a dazzling fitted white linen blazer from high-street favourite Topshop and a pair of coordinating white stilettos. Perfection!
She wore her flowing blonde locks in cascading waves and finished off her wedding guest attire with an alluring Manolo Blahnik clutch adorned with giant white polka-dots.
As well as Downton creator Julian Fellowes, stars Lily James and Elizabeth McGovern, a whole host of other celebrities were also in attendance including the groom's older sister, Fleabag and Indiana Jones star Phoebe Waller-Bridge, and The Crown actress Lesley Manville.