20-year-old Mariah Yeater filed a paternity suit last week claiming to have fallen pregnant after sleeping with Bieber at a California concert in October 2010.
The singer has pledged to take a paternity test to prove that he is not the father of her three-month-old son Tristyn.
George, who along with his bandmates supported Bieber on his South American tour, said that he is providing moral support to the 17-year-old amid "the worst time of his life".
"He is really cut up by being accused of being a father," George told The Mirror. "He said it's absolute bulls**t lies and that it didn't go down like that at all.
"I told him to keep his head up, we're right behind him. He knew we'd understand because girls are around us all the time, too. He's had the worst time of his life and handled it so well. Doing the paternity test shows how grown-up he's being about it."
George added that he was confident to back "adopted brother" Bieber over the claims, as he had always seen the young star as innocent and fun-loving.
"Justin is like our adopted little brother, that's why I believe all this baby stuff is rubbish," he said. "Any spare time Justin had on tour, he'd joke around - crawling under seats and grabbing girls' ankles. It was hilarious.
"Justin turns 18 in March, so we're going to corrupt him. I'm talking a proper British party with loads of ale and killer shots. What are mates for?"
George recently claimed that Bieber "deals with the pressure" of fame well.
Watch The Wanted in the video for 'Lightning' below: