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In a post on Instagram on Saturday, the X Factor presenter announced TV producer Dee had given birth to their son on Tuesday.
Dermot posted a picture of a babygrow with “Koppang O’Leary Productions Est. 2020” written on it.
He wrote: “Welcome to the world baby Koppang O’Leary.
“We’re delighted to announce that we’ve had a baby! A little boy born on Tuesday 23rd June 2020 (Sankthansaften / Midsummer for you Norwegians) at 8.19am, weighing in at 6lb 13oz.”
Dermot added that he and Dee were “enjoying the cuddles in the newborn bubble”, but said their pet cats are “yet to be 100% convinced”.
Dee also shared the same photo with the same caption on her Instagram account.
It is the first child for the couple, who married in September 2012.
The pair announced they were expecting back in February.
Former Strictly Come Dancing winner Ore Oduba was among the first to offer his congratulations to the couple on Instagram, commenting: “Delighted!!! Absolutely over the moon for you all.. CONGRATULATIONS Koppang O’Learys!!!!”
Presenter Rochelle Humes said: “So SO happy. Congratulations to you both, big love.”
This Morning host Holly Willoughby added: “Congratulations.... love you both... welcome to the world baby Koppang O’Leary, can’t wait for snuggles.”