Jennifer Garner is leaning on her boyfriend John Miller following her ex-husband Ben Affleck's ''slip'' in his sobriety.

The 'Justice League' star had spent a year away from alcohol before he admitted to having slipped up during a Halloween party last weekend.

And Jennifer - who has been supportive of Ben throughout his sobriety journey - is said to have been hit hard by the news of his relapse, so is now relying on her new beau John to be her ''rock''.

A source told Us Weekly magazine: ''[Jennifer] knows she can rely on [John]. They are very happy, and going strong. John has been a rock for Jennifer, especially during Ben's recent relapse.''

The news comes after it was recently reported the 'Peppermint' actress, 47, is still ''supporting'' Ben, but is ''sad'' to see him relapse.

Another insider claimed: ''She's upset but always supportive.''

Ben, also 47, was believed to have relapsed when he was seen stumbling and falling against a car after attending the UNICEF Masquerade Ball at the Kimpton La Peer Hotel over the weekend.

Following the sighting, the 'Argo' star - who has daughters Violet, 13, and Seraphina, 10, and son Samuel, seven, with Jennifer - told reporters he wouldn't let his slip up ''derail'' him from his sobriety journey.

He said: ''Well, you know, it happens.

''It's a slip, but I'm not going to let it derail me.''

The slip-up and his subsequent comment came just after Ben joked about headlines focused on his private life, and then pledged to donate to homeless charity The Midnight Mission.

He wrote on social media: ''HA, you got me. I'm dating. But let's be serious for a moment and talk about something that is actually important.

''I have been in recovery for over a year and part of that is helping out others. @themidnightmission is an incredible organisation that helps those in need with housing, training, development and recovery.

''I'm making a donation today because there are people battling addiction every day that don't have the resources and need help. @raya, you in? Who else is with me? (sic)''