The singer, 43, allegedly arrived at London's Moorfields Eye Hospital and told medical staff that she was 'totally blind' and 'couldn't see anything' after losing vision in her right eye, reports The Sun.
Mel is already unable to see out of her left eye due to botched laser surgery, which took place 20 years ago.
A source told the website: 'Mel was in a really bad way. The other girls and their managers were really worried.
'Mel was frantic and screaming that she couldn't see a thing. She went to the hospital in a chauffeur-driven car with an assistant, who was holding her hand.
'No one knew what brought it on but she had very swift, and very good, treatment.'
The insider suggested that it is currently unclear whether Mel's alleged loss of vision was linked to her previous problems with bungled eye-surgery.
They added that after arriving at the hospital, Mel was later transferred to Luton and Dunstable Hospital's ophthalmology department and left once she could see again.
A spokeswoman for Mel told the publication: 'The situation with Mel is under control. She is looking forward to the tour.'