Jorge Lorenzo is set to take over the now vacant seat at the Repsol Honda MotoGP team next year.
Lorenzo had originally been pencilled in to ride for a new Yamaha satellite team owned by Sepang International Circuit, which already runs in Moto2 and Moto3.
Lorenzo's and Petrucci's moves all but guarantee that Jack Miller, who had also been linked to Repsol Honda, will stay at Pramac.
The Australian would, however, be given the latest-spec Desmosedici GP19 next year when he would be joined by the already confirmed signing Francesco Bagnaia.
Pramac currently runs Petrucci on a GP18 and Miller on a GP17.
The pairing of Marquez and Lorenzo, who won onboard a Ducati for the first time last weekend, would make for a powerful combination.
In only one of the preceding eight seasons has another rider won the MotoGP world championship, namely Casey Stoner in 2011 when he too was a Repsol Honda rider.
Lorenzo won his first premier class title in 2010, his second in 2012, and has been the only man to deny Marquez in the past five years when he bagged a third in 2015.
Movistar Yamaha, whom Lorenzo rode for on each of those occasions, is likely to be the only factory team which will take an unchanged rider line-up into 2019.
Red Bull KTM will field Pol Espargaro and current Tech3 rider Johann Zarco while Suzuki has re-signed Alex Rins and will likely replace Andrea Iannone with Moto2 up-and-comer Joan Mir.