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Holly, who has been on the show for four years, and Phillip, who was marking his ten years, reveal how behind the glitz and glamour, the party felt like one big family get together.
"It's a real family affair, that's what we are," said Holly. "You feel guilty saying, 'I'm going to work' when actually you're going to hang out with one of your best friends."
"Holly's like my little sister," said Phillip. "We've never had a cross word, never disagreed and our sense of humour is the same speed. It's a lovely way of working."
As celebrities arrived for the party at London's Home House, the show's original presenters Richard Madeley and Judy Finnegan, and their children Jack and Chloe, reminisced on how their family had grown up with the show.
"Even now when I hear the This Morning theme tune, it's like my childhood," Chloe told HELLO!. "It was a huge part of growing up. Dad said, 'You're invited to the This Morning party, and we want you to come, you were a big part of our time there.'
"I was less than a year old. I was on the first show getting the MMR jab, had my first swimming lesson on the show, my first holiday, my first hair cut.
"People still come up to me and hug me and say how they watched me grow up and how they adore mum and dad. Mum doesn't like being recognised so much but dad still loves it."
Just as Judy joined the show when Chloe was a young baby, so Holly's two children, Harry, 4, and Belle, 2, are growing up with This Morning.
"I started in September 2009 after I'd just had Harry," said Holly. "It's actually the perfect show when you have a young family. I get in at half past seven in the morning so I can see them in the morning and pick Harry up from school."
The party was a double celebration for Matt Johnson who had just clocked up three years on the show. "It's lovely that they've laid on this big party for my birthday," quipped Matt whose 32nd birthday fell on the same day.
"It's a massive thing to be part of the This Morning family," he added as he tucked into cakes and sweets from the Vintage Sweet Shop. "The secret to our success is that we're all so close, the crew, the talent, Phil, Holly, we're all mates and try to socialise as much as we can."
Other star members in attendance included agony aunt Denise Robertson and resident doctor Dr Chris Steele, who have both been on the show since the beginning.
Atomic Kitten's Liz McClarnon and Liberty X's Michelle Heaton, who at six months pregnant wasn't able to indulge in the drinks provided by I heart wine, Heineken or Caorunn Gin, reminisced on the show's success through the years.
"This Morning is an institution," said Michelle. "I've watched all different presenters through the ages and now with Rochelle and Marvin there is a new generation of presenters coming through, it's really exciting."
As the hosts past and current were presented with a flamboyant Choccywoccydodah cake, all involved sang a rousing rendition of "Happy Birthday".
Guests danced the night away to sets from The Overtones, Stooshe, The Risk and tunes by DJ Adam Fabulous, and captured the night with snapshots from photobooth.co.uk. A giant 25th birthday sign had been made especially by Hamilton Ice Sculptors for the party.
Before heading home, guests were treated to goodie bags jam packed with gifts from Boots, MisFit Shine, Baylis & Harding, Max Factor, Vapestick and PreFunc.
Read the full story and see more pictures in this week's issue of HELLO! on sale now.