The music concert, which is being organised by BBC Radio 1, will take place on the weekend of June 23-24 at Hackney Marshes.
The group join a lineup already consisting of Leona Lewis, Plan B and Tinie Tempah. The show marks the run-up to the London 2012 Olympics.
It will be the biggest free-ticketed event that Radio 1 has ever held, with over 100,000 people able to attend across the two days.
The Big Hackney Weekend will play host to six stages which will be graced by over 80 different acts from all types of genres.
The festival will be broadcast on Radio 1, 1Xtra and BBC Three throughout the weekend.
Details on how to apply for free tickets will be announced in the New Year.
Florence + the Machine recently topped the UK album chart with their new LP Ceremonials.
Watch the music video for Florence + the Machine's new single 'No Light, No Light' below: