Pietersen's place in next month's Trent Bridge opener had been placed under some uncertainty after he took longer than expected to recover from a knee injury that flared during the tour of New Zealand in February, but the right-hander blasted his way to a near run-a-ball ton for Surrey in their LV= County Championship match against Yorkshire at Headingley.
England managing director Hugh Morris said he was confident Pietersen would be ready to face Australia in the first Test at Trent Bridge, which begins on July 10, and he told BBC Radio Five's Sportsweek programme: "It's very encouraging that Kevin is back playing again. He looks very fit."
He added: "I know he's worked very hard in practice on preparation and on his fitness. Clearly it's important that he gets some cricket under his belt before the selectors actually sit down and pick the first Ashes squad.
"We've got a few weeks yet and he's got this four-day game and he's in our Twenty20 squad for New Zealand and we've got a game against Essex leading into the Ashes, so there is some cricket left, which is encouraging."
Pietersen reached his century from just 106 balls - and went on to hit 17 fours and seven sixes - on the same day Australia opted to call up Steve Smith to their Ashes squad as cover for injured captain Michael Clarke.
Pietersen will remain with Surrey on Monday and, provided he suffers no setback from Sunday's innings, is due to make his England return in Thursday's second Twenty20 international against New Zealand at The Oval.
Before Sunday's play Pietersen had written on his Twitter account that his knee had felt fine after Surrey had been forced to field during the first two rain-affected days.
He wrote: "And by the way - after fielding for 136 overs, my knee feels top drawer!! All smiles this Sunday!!"