Jessica Shepherd, who needs £120,000 to fund treatment in America , named her tumour after the ex-Mayor of London in jest.
Her family went through a list of names to call the tumour, and 'Boring Boris' made the little girl laugh, and so the slogan 'Help Jess Beat Boris' was born.
Jess was first diagnosed with a rare form of childhood cancer, neuroblastoma, aged just three years old, and named her tumour Boring Boris, when it kept returning as it: "won't go away and keeps coming back”.
He told getwestlondon: “Jessica has shown tremendous courage in her battle against cancer and I hope people will support this fantastic fundraising effort.”
Regardless of politics, 'he's a really nice man'
The MP's support didn't end there however, and Jess was invited to meet him at the Civic Centre in Uxbridge , on April 22.
They posed for a picture with Jess hitting Boris Johnson over the head with a blow up hammer, in reference to their slogan 'Help Jess Beat Boris'.
Jess's nan, who does not wish to be named, said: “She had great fun hitting him with the blow up hammer!
“He was so nice to her, regardless of anyone's politics, he was a really nice man.
"Jess really liked him and he's written to her since to check on her progress so it wasn't just a one-off."
In order to help raise funds needed to further Jess's treatment, her home village of Harefield came together with a 'Disney Day' fundraiser, complete with prince and princess fancy dress.
For more information on events to help Jess and to donate, visit their crowdfunding page .