Justin Timberlake appears to have lost hundreds of fans after using 'racially charged' language when wishing his good friend Madonna happy birthday on social media.

The pop star found himself in hot water after tweeting the 56-year-old singer on her birthday saying: "A HAPPIEST of Bdays to my mother chucking ninja, @Madonna!! Hope you have a great one, M!"

Within minutes, Twitter users expressed their outrage, pointing out the phrase "mother chucking ninja'' was a common replacement for the n-word.

Many also claimed the careless tweet was distasteful seeing that tensions continue to mount in Ferguson, Missouri, following the fatal shooting of unarmed teenager Michael Brown by police.

"I just ended my relationship w/Justin," one unimpressed fan wrote.

The Sexy Back hitmaker and Madonna have been good friends since working together on the song 4 Minutes in 2008, which featured on the Material Girl's Hard Candy album.

Although the 33-year-old star has not yet publicly apologised, he quickly removed the offending tweet after a barrage of complaints.

He later retweeted a message about the events in Ferguson: "The world is watching #Ferguson. [And we pray.]"