Jessie J grateful for Wireless performance

Jessie J didn't think she could "bounce back" from her illness in time for Wireless Festival.

The British singer was on stage in London yesterday despite having only just been released from hospital after surgery for an undisclosed illness.

She told the crowds her doctors had advised her not to perform and took to Instagram to reveal how touch and go it was whether she would be there.

"When you feel like giving up... DON'T

"I felt so lucky to be on stage today | If you had seen me last week | No one in my friends and family thought I would bounce back for this show (inc myself) | It took everything I had but I did it | I did it | Thank you #wirelessfestival (sic)," she posted.

The 27-year-old had her mother, nephew and brother along for support and gave the show her all. Wearing a black high-waisted bikini and breezy white cover-up, she belted out her hits despite warning she wouldn't be on top form.

"I'm out of hospital | Still very fragile but much better | I will be at Wireless tomorrow | Not my usual crazy running around self yet but I will be there | Can't wait (sic)," she told followers on Twitter on Saturday.

There's been no word on what Jessie was suffering from but it's known she has a heart condition, which means she's struck down by illness fairly regularly.

In March she was forced to cancel shows in Australia due to the heart problems, which she's had since birth.