Robbie Williams and wife Ayda Field surprise fans after welcoming "secret" fourth child

Congratulations are in order for Robbie Williams and his wife Ayda Field, who have welcomed their fourth child!

Taking to Instagram over the weekend, the singer's wife made the surprise announcement in a heartfelt post.

Alongside a photo of four sets of children's feet, one of which was a baby's, Ayda wrote: "Spot the difference... On this Valentine's Day, we would like to celebrate love in the most awesome way ... Beau Benedict Enthoven Williams."

She went on to reveal the couple welcomed their son via surrogate.

"As with Coco, he is biologically ours, but born via our same incredible surrogate.

"We are so blessed to have our healthy son safely in our arms and are officially complete as a family #happyvalentinesday #surprise #familyof6 #whodathunkitinthe90s The Williams Family xx."

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Ayda and Robbie had not revealed that they were expecting a fourth child.

The pair's daughter Coco, one, was also kept a secret until her arrival last September. Following her birth, Ayda explained on Instagram that she and Robbie endured a "long and difficult path" trying for another baby.

She wrote: "Family comes in all forms, and this little lady, who is biologically ours, was carried by an incredible surrogate mother, to whom we will be eternally grateful."

The couple married in Los Angeles in 2010 after dating for four years. They also share a daughter Theodora (Teddy), seven, and son Charlton (Charlie), five.