Some 45 million poppies, made at a factory in Richmond, Surrey, will be distributed nationwide and go on sale on Saturday.
The fundraising was launched with a concert at Trafalgar Square, featuring military bands and pop singers. Poppy Scotland, a sister charity, also launched its annual poppy appeal.
The British Legion said a poppy can be worn at any time, but they are usually available two weeks before Remembrance Sunday. The legion's head of fundraising events, Theresa Greener, said people wore poppies for different reasons. "The poppy has never been about a political statement. It is a symbol of remembrance. It's broadly worn across the whole community regardless of age, regardless of background or culture.
"I think people wear their poppy for myriad different reasons actually. There are many people they want to remember, either generically or specifically. We don't want to dictate how people should wear their poppy, we're just very grateful that they do it."
About 350,000 volunteers will help with the legion's appeal, selling the poppies at various locations, including train and tube stations and shopping centres. Last year's appeal raised £40m.
Jade Mandara, the partner of a serving soldier, described the legion as "angels" after they helped her with rent and household goods when she had to suddenly move accommodation while her partner was serving in Afghanistan.
"They've been a true support for me and my family while my partner's been away. So, they are my angels. I don't think people are aware of where their money goes when they donate to the poppy appeal. There are families like myself, the younger generation - not just the older generation, the World War Two veterans. It's a wide span of people that need help in the army... family."
The free London concert featured the Military Wives Choir and James Fox, star of the West End musical Let It Be, who will sing the official Poppy Appeal 2012 single Landlocked.
Singer Alesha Dixon will perform with The Corps of Drums of the 1st Battalion Coldstream Guards and 1st Battalion Irish Guards, while singer Pixie Lott will appear alongside trumpeters from the Band of the Grenadier Guards.
The Royal British Legion said it spends £1.7m a week on care and support for military families, including grants, employment advice and funding, emotional support, tribunal and inquest advice, care homes and family breaks.
Poppy Scotland launched its fundraising event, with an appearance by singer Susan Boyle. It raises £2m each year to support service personnel and their families in Scotland.