The stars met on this year's 'Strictly Come Dancing Live Tour' and struck up a friendship so, when Rachel was asked to host the event, she contacted Mark to see if he'd like to join her.
Mark said: "Rachel told me all about the Butterfly Ball and asked whether I would like to get involved. It sounds like an unbelievable event and I'm just pleased that I can help such a worthy cause."
The Butterfly Ball is the flagship fundraising event for Caudwell Children, a charity which supports disabled children and their families. Several celebrities are patrons including Peter Andre, Liz Hurley and Robbie Williams.
This year's event takes place on Thursday, 25 June at Grosnevor House Hotel in London, with Lionel Richie as the special guest performer. Other performances will come from Paul Potts and Tito Jackson.
Rachel recently spent three days working with the charity on their Destination Dreams project, which takes unwell children and their families off to the sunshine for a holiday.
"To see the families enjoy this really special time together, and to see the difference it makes to them was very special. It inspired me to do whatever I can to help so when I was asked to host their annual Butterfly Ball, which I've attended as a guest many times, of course I said yes," she said.
"This year will be even more special to me having spent time with some of the families who will be there and I'm looking forward to working with Mark and to helping the charity raise funds to help even more people across the UK."
For more information about Caudwell Children and The Butterfly Ball visit www.caudwellchildren.com/butterflyball
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