According to an Us insider, Sorrentino was "downing so many cocktails, he could barely stand" while at the London hotel in West Hollywood on September 26.
The source reportedly attributes Sorrentino's alleged current drinking problem to the cancellation of Jersey Shore. MTV confirmed in late August the upcoming sixth season of the reality show -- which premieres Thursday, October 4 at 10PM ET/PT -- will be its last.
"Mike's freaking out about what he's going to do after the show," the source told the magazine.
Sorrentino had finished rehab in early April after entering a treatment program for an unspecified substance abuse problem. RadarOnline reported at the time the reality TV star was in rehab for prescription drug dependency.
News that Sorrentino had been seeking treatment first surfaced in late March.