Kelly Osbourne has mocked Christina Aguilera for being "fat".

The 26-year-old star has reignited her war of words with the 'Dirrty' singer during an appearance on E! TV show 'Fashion Police' while commenting on the unflattering outfit Christina wore at the Michael Jackson tribute concert in Cardiff, Wales, earlier this month.

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."

To which Kelly replied: "She called me fat for years. I was never that fat."

Describing Christina's unflattering figure-hugging outfit, fellow 'Fashion Police' host Joan Rivers cuttingly said: "Lady Marmalade got into the peanut butter again."

This is not the first time Kelly has attacked Christina over her seemingly ever-increasing weight on 'Fashion Police'.

In August, the blonde beauty described the 30-year-old pop star as a "fat bitch" while describing a tight black dress she was pictured in.

Kelly - who has shifted a considerable amount of weight this year after embarking on a new healthy eating and fitness regime - said: "Maybe she's just becoming the fat bitch she was always born to be. I don't know.

"She called me fat for so many f***ing years, so you know what? F**k you, you're fat too."