According to The Hollywood Reporter, the special was to be filmed at the same studio where the sitcom was shot, Stage 24 of the Warner Bros. Studio in Burbank, California.
The special was to be unscripted and was billed as a celebration of the series.
A source told The Sun on Sunday newspaper: "This reunion has been 18 months in the making. Cast and crew have been made to sign rigid non-disclosure agreements to keep everything as fresh and exciting as possible.
"It took a long time to convince Matthew and Jennifer in particular to get on board. But once these two were signed up, things moved pretty quickly."
The proposed 12-part series could be worth as much as $1 billion.
The insider added: "Officially, the deal has been signed for a one-off. But the hope is that once everyone sees how well it's received and how much love there is for the show, another series will be agreed and commissioned.
"If this happens, agents will be licking their lips. This will be the kind of small-screen payday the likes of which has never been seen. Including streaming rights, cast fees, spin-offs and merchandising, this could be TV's inaugural billion-pound contract."
The Friends reunion was recently confirmed by the star-studded cast via social media.
They all shared a picture of themselves together, and posted the same caption, which simply stated "it's happening" and tagged their co-stars, and HBO Max.
The sitcom - which first aired from 1994 until 2004, lasting ten seasons - was one of the most popular TV shows of all time, winning numerous awards during its original run.
