Powell, who joined the London Flower Show team in March to provide creative consultancy and support to the team on their show plans and content, has been working on a freelance basis with the organisers Live Industry to plan and develop content for this year's show.
Over the past three months Powell has given extensive professional advice and helped the team to develop a new look and a number of fresh new concepts to launch this year's event. She has now stepped down and handed over to event organisers to deliver the show's plans to visitors this September.
A spokesperson for Powell said the "direction of the event changed", leading to Powell's decision to withdraw from the role.
Powell said: "Being involved in the early stages of the London Flower Show has been fantastic. I have really enjoyed my time as part of the team, and I hope that my experience, creative advice and knowledge of live events has provided the team with some valuable ideas and content.
"I wish the team well in their endeavours and I hope that they can deliver a fantastic event that is a huge success for visitors this September."
Managing director of Live Industry Samuel Speight said: "Ann-Marie brought with her an outstanding portfolio of work and experience and is brimming with creative ideas. It has been a pleasure to work with her and we will be sure to utilise her advice and conceptual ideas to create a memorable event in 2016."
The London Flower Show is set to run from 8-11 September 2016, within the grounds of the Georgian country estate Osterley House and Park.