The couple are said to have become close after working together for more then ten years, and have now confirmed their relationship to family and friends.
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They have allegedly also told their bosses at celebrity agency James Grant Management, the company where Ali works and which oversees both Dec's career and that of his on-screen partner Ant McPartlin.
"They adored each other for years and people often thought they were together — but it's only recently they finally became an item," a source told the Sun.
"They kept it quiet for ages but are at last coming to terms with being a couple in public."
On Saturday, Dec, 37, and Ali, 34, were seen arriving back in the UK together after a holiday in LA.
They are said to have enjoyed a series of secret dates for "several months" and have regularly been spotted out and about together in Chiswick, west London, where they both live.
Declan pictured with former girlfriend Clare Buckfield
"They have told their families the news as they all know each other anyway," the source added. "To be honest, it hasn't come as much of a surprise to some because they'd always been close."
Dec — who co-hosts I'm A Celebrity... and Britain's Got Talent — has not had a serious romance since splitting from Sky Sports host Georgie Thompson in 2011.
Prior to that, he had an 11-year relationship with 2point4 Children actress Clare Buckfield which came to an end in 2003.
He was also briefly linked to Pussycat Dolls star Ashley Roberts, who was runner-up in the 2012 seres of I'm A Celebrity.