Long-time friends Kate Winslet and Leonardo DiCaprio "speak on the phone constantly".

It's been more than 18 years since Kate and Leonardo met on the set of Titanic, and while their first onscreen relationship ended tragically, the pair's affection for one another has lived on in the years since.

The two stars, who shared the big screen again in 2008's Revolutionary Road, have remained close friends, with Kate, 40, shedding tears of joy when Leo collected his first Oscar for his role in the film The Revenant last month (Feb16).

Now, a source close to the actors reveals the pair maintain their friendship by supporting each other whenever possible.

"Kate and Leo had a brother-and-sister chemistry from the start," an insider tells Britain's Heat magazine. "They've always been there for each other. They may not always be in the same country, but they speak on the phone constantly."

The source added that the duo had always brushed off suggestions that they should be a couple, prior to Kate marrying Ned Rocknroll in 2012.

"They both know if they'd ever slept together, they couldn't have the relationship they now do. Their friendship survived this long because they've avoided that," the source revealed.

There may not be romance but there sure is a great deal of mutual respect between the pair and Leo, 41, enjoys spending time with Kate's three children.

"Kate doesn't judge him, and Leo is great with Kate's kids, Mia, Joe and Bear - they call him Uncle Leo. It's the perfect relationship," the source explained.

After ending his two long-term relationships with Gisele Bundchen in 2005 and Bar Rafaeli in 2011, Leo has been linked to a long list of eligible ladies including models Erin Heatherton, Toni Garrn and singer Rihanna.