Kelly Osbourne is going after Christina Aguilera, mocking her for being fat. Aguilera's weight gain was apparent during her performance at the Michael Jackson Tribute Concert.

Osbourne, 26, slammed Aguilera on Friday's E! Online episode of Fashion Police. She called me fat for years, Kelly said. I was never that fat.

Osbourne escalated her war with Aguilera by branding the singer a fat b***h.

Fashion Police host Joan Rivers, in her signature style, described a flabby Aguilera as Lady Marmalade got into the peanut butter again.

Co-host George Kotsiopoulous said, I'm looking at this and thinking, 'Well, she's put on weight. This just isn't flattering.' But this is just a crappy outfit, regardless. No matter what shape your body is in, it's just bad.

But when Kotsiopoulos tried to defend Aguilera by commenting that she was still probably a size 2/4. Osbourne disagreed and said, Trust me. I'm a 2/4. That is not a 2/4.

This is not the first time that Osbourne has insulted Aguilera. In August, when Rivers commented that Aguilera looked as if she is stuffed into her Givenchy dress, Osbourne added that Maybe she is just becoming the fat b***h she was born to be. I don't know. She was a c***t to me. And she bought my house! ... She called me fat for so many f***ing years, so you know what? F*** you! You're fat too.

Osbourne got even with Aguilera for past verbal abuses and pointed out that, I took more hell for being fat than I did for being an absolute raging drug addict. I will never understand that.

Although Osbourne shed 48 pounds in 2009 and kept it off, the memories of people criticizing her for being overweight still sting.