Jane Tomlinson has already completed two London Triathlons

The 38-year-old mother of three from Rothwell, Leeds, will swim 1.2 miles, cycle 56 miles and run a half-marathon on 31 August at Sherborne Castle, Dorset.

She has cancers throughout her body and suffers pain and disorientation.

Mrs Tomlinson is trying to increase the �312,000 she has already raised for cancer charities.

She said: "Physically and mentally I know I can do it but it's just on the day it is so difficult to stay mentally with it.

"If the weather is like it has been in the heat wave, it will be hard."

Bike ride

Her husband Mike said: "She has cancer sites in her shoulder, back, spine, leg, thigh, hip and knee as well as her lungs and bones.

"Every time she puts her foot on the floor, it sends pain, similar to a stress fracture, to her thigh.

"Every time she moves her arm, it sends pain to her shoulder."

During the triathlon she will have to swim backstroke so she does not become disorientated in the water.

Earlier this month, Mrs Tomlinson finished the London Triathlon for the second time.

She has also run two London marathons and cycled with her brother Luke from John O'Groats to Lands End.