Jessie J hits the stage in London after being hospitalised with mystery illness

Jessie J performs at the New Look Wireless Festival at Finsbury Park in London. Source: Getty Images

JESSIE J is fighting fit after being admitted to hospital with a mystery illness last week.

The British pop star hit the stage on Sunday night for day three of London’s Wireless Festival in Finsbury Park.

The Voice Australia coach delivered an energetic set, performing hit songs from her current Sweet Talker tour.

Rocking an edgy black ensemble of a crop top and high-waisted pants with her midriff showing, the Price Tag singer looked to be in fine form as she belted out her chart-topping numbers to the delight of the crowd.

Jessie J performs on the main stage at Wireless festival.Source: AP

Jessie J on day three at Wireless festival. Source: AP

The 27-year-old took to Twitter the previous day to reassure fans she would be performing at Wireless, despite still feeling “very fragile”.

I'm out of hospital 🎉| Still very fragile but much better | I will be at Wireless tomorrow | Not my…https://t.co/HS1eXU8MHi

— Jessie J (@JessieJ) July 5, 2015

The Nine Network assured concerned Voice fans that their newest coach would be returning to Australia in time for the live shows in the coming months, despite the singer keeping a low profile over the last two weeks while in hospital with her undisclosed sickness.

“We are sending our very best wishes for a speedy recovery to Jessie. We do not anticipate her illness impacting her return and we look forward to welcoming her back for the show in the coming months,” Nine Network spokeswoman told Contactmusic.

Jessie J responds to her fans after concerns over her mystery operation. Source: Instagram

The Bang Bang singer posted photos on social media of herself hooked up to a dripin hospital last week, revealing she had an operation but would not divulge any more information.

“I am not OK ... I am in a lot of pain.

“But I’m working on it with the doctors.