Miley Cyrus "deserves to have fun" at her 19th birthday party, says Kelly Osbourne.

Kelly is planning the big bash for her friend, which is set to kick off tomorrow.

The 27-year-old wants to make sure Miley enjoys herself and takes a break from working.

"Although I see her on the same level mentally as me, I also realise that Miley's 19 and she deserves to still have fun," she told E!

"Because that girl has so much pressure on her and has worked her ass off her entire life that I think a lot of the time people forget that she deserves to have fun too. So that was my mission for her, to make her birthday... the best day ever."

The pair became friends while filming the new movie So Undercover. Just last month the singer helped Kelly to celebrate her birthday at the Little Door restaurant in Los Angeles.

The reality TV star is delighted to be able to return the favour.

"It's intimate, but it's going to be the sickest thing she's ever seen in her life," Kelly exclaimed. "I've got all sorts of madness and craziness."

However, Kelly is remaining tight-lipped on the details of pal's big day.

Although she did reveal that she has been planning the event for a long time and she was working until 11pm at the venue on Monday to try and get all the arrangements sorted.

"busy busy day of planing my dear friend at MileyCyrus birthday party! this will be one to remember !" she tweeted on Monday.