Stokes had scans on his injured left knee on Thursday night - and although they were inconclusive, after his partial involvement in net practice at the Ageas Bowl on Friday morning, an ECB spokesman reported the key all-rounder will be fit to both bat and bowl as England seek an unassailable 2-0 series lead.
A brief statement confirmed Stokes "is fit to play and is available to bowl".
Stokes managed just two overs at Headingley in England's opening victory because of pain in the same knee which needed surgery last summer and ruled him out for two months.
England were hoping their number one match-winner would be able to play a full part in training in Southampton.
He did have two long net sessions as a batsman but then, in two attempts either side of adjusting the strapping on his injury, bowled only around 12 deliveries into the glove at the edge of the square.
Confirmation that he will be available for selection on Saturday nonetheless arrived earlier than expected just as England left the ground after practice.
England seamer Liam Plunkett had spoken shortly beforehand about Stokes' importance to the team but his confidence too that others could step in if needed.
"If it was up to him, he just wants to play every game," Plunkett told Sky Sports News."If you didn't have him in any team, you'd be disappointed. He's a massive player who can win games with bat, ball and in the field.
"But we've also got other players who can come in and win games - as we've shown. We've got people to cover all bases. If Jonny [Bairstow] comes in as the batter, I'm sure he'll perform; if not the other bowlers will perform too."
England have begun this series with increased security measures promised around the match venues after Monday's terror attack in Manchester.
Plunkett pointed out it is the players' duty to clear their minds and put on a show. Asked about the unsettling situation, he said: "It's everywhere now, all around the world, so you've just got to crack on with what you do.
"We want to perform, get people in to watch the game and have a fun day."
