Kate Middleton was rushed to hospital by Carole Middleton: 'very serious operation'

Kate Middleton, who has recently revealed to finish her course of preventative chemotherapy, once left her mother in deep water. 

Prince William's wife Kate had avoided hospital for most of her life until recently, she did have one previous health scare which reportedly made her mother 'very worried'.

When she was boarding at Marlborough College she discovered a mystery lump on the left-hand side of her head. Carole took her daughter to see their family doctor who considered the lump 'potentially serious', according to royal author Katie Nicholl.

The future princess was operated on within a few days, but the surgery left a three-inch scar on Kate's head.

A housemistress at the school during Kate's time there, Ann Patching, told Nicholl: 'I can remember the incident and her having an operation. I don't recall anything happening on the hockey pitch (field) that had anything to do with the lump.'

She said after the media reported that the health scare might have been the result of a sporting accident.

According to Mail, Nicholl writes in her 2013 book, "Kate: The Future Queen," Mrs Patching told her: 'Catherine had the operation during her term time. She was back at school very soon afterwards. As usual, nothing was too much of a big deal for her. You could never accuse Catherine of being a drama queen, but Carole was very worried, as any mother would be.'

A former pupil told Nicholl that the operation 'was pretty serious' and alarmed everyone, as it had happened shortly after another pupil, Hugo McDermott, had died from a brain tumour.

But following the operation, Kate's scar began to fade and usually it is hidden as she tends to wear her hair loose.

The blemish was first spotted in 2011 when the princess was carrying out her first solo royal engagement at a black-tie private dinner at Clarence House.

An official statement explained that 'the scar related to a childhood operation', but said that details of the surgery were a private matter.