Pop star Prince's medical emergency has been revealed as "bad dehydration".

The 57-year-old was aboard a flight when he fell ill, forcing it to make an unscheduled landing in Illinois on Friday morning.

TMZ reported the singer's plane diverted to Quad City International Airport in Moline to make the emergency landing.

A source close to Prince told the Star Tribune - the newpaper in Prince's hometown of Minnesota - that he had been suffering from "bad dehydration".

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He was "all good" now and was back home fighting the flu.

Prince had been battling the flu for several weeks, TMZ reported.

The Purple Rain hitmaker had performed at The Fox Theatre in Atlanta on Thursday night - the second of two back to back sold out shows - and had seemed fine during the concert.

Fans uploaded snaps to Instagram of him playing, with no apparent difficulty.

But after leaving Georgia on a private jet about 11pm, he was only in the air for about two hours before making the unscheduled stop at 1am.

TMZ reported he was feeling unwell when he boarded the plane and felt worse after taking off.

He was hospitalised after the emergency landing but returned to the plane to fly home three hours later.

He made his last post to his Instagram account - or Princestagram as he calls it - on Thursday: a black white screen.

But his 214,000 followers won't look at this as an omininous sign, as he - like Jaden Smith - does this all the time, possibly to frame the photos that remain.

- Additional reporting: NZ Herald