Wasps flanker Haskell missed the opening two victories over Italy and Wales through suspension, as did Harlequin prop Marler, but both are in the 25-man squad to face Scotland.
England have also been boosted by the return of Wasps number eight Nathan Hughes, who has been hampered by two knee injuries and has only played 90 minutes of rugby in the past three months.
"Everyone's fully fit for selection this weekend," forwards coach Steve Borthwick said.
"Nathan's worked really hard, really diligently and he looks in really good shape. He's healed very fast which is great.
"While our strength and conditioning and medical team have worked incredibly hard, the credit has to go to Nathan for the work he's done while he's been injured."
Hughes will come in for Sam Simmonds, who has been ruled out with a shoulder injury.