The actress and musician will be raising money for Papyrus and Alzheimer’s Research UK.
The announcement comes after Carrie participated in last year’s Vitality 10K and took up running in her spare time.
Sharing the news in a post on Instagram, Carrie said: “When I say that the Vitality 10K I ran back in May 2019 was a challenge, I mean it because I am not a runner nor do I find running easy by any means – which is why my face looked like that when I actually passed the finish line!
“So saying YES to running the 2020 London Marathon felt like such a HUGE thing. I may not be the fastest or the strongest and I may crawl over the finish line but I can promise to be the most enthusiastic and the most determined.
“I will get the A* for effort and I will try my hardest to grin my way around the 26.2 miles,” she added.
So…I'm running the London 2020 Marathon… pic.twitter.com/4Y15I2Lks6
— Carrie Hope Fletcher (@CarrieHFletcher) January 7, 2020
Carrie was one of thousands of people who successfully secured a spot in the run when the ballot results were announced in October.
Over 457,000 people registered for a place in the April event, with more than 373,000 applications from the UK.
Writing on her fundraising page for the marathon, Carrie has described the run as “[her] biggest challenge yet”.
“I can’t promise to be the fastest or the strongest but I can promise to be the most enthusiastic, the most determined and to smile the entire way round (even if I feel like my feet fell off in mile three!),” she writes.
Over £210 has already been donated online, which is 4% of Carrie’s £5,000 target.
Donations can be made online via the Virgin Money Giving website.