While Cricket Australia maintained it was an alleged "physical altercation," its English counterparts used stronger language by saying Warner "initiated an unprovoked physical attack" in a Birmingham bar following England's victory over Australia in their opening Champions Trophy match on Saturday.
"Warner has admitted behaving inappropriately and has since apologized to the player involved who has accepted the apology," the England and Wales Cricket Board said in a statement.
The ECB said it concluded Root "was in no way responsible for nor retaliated to the attack."
Warner — a left-handed opener and one of the cornerstones of Australia's batting department — faced a Code of Behavior hearing via teleconference with Cricket Australia officials later Thursday and could be sent home from the tour of England.
Cricket Australia has reported him for engaging in "behaviour unbecoming to a representative player or official that could (a) bring them or the game into disrepute or (b) be harmful to the interests of cricket."
Australia batsman George Bailey, who has been standing in as captain for the injured Michael Clarke, said the incident was "very minor" and defended Warner's character.
"I really enjoy playing cricket with him — love his enthusiasm, his energy around the group and I love the way he plays. I wish I had the talent he has," Bailey said. "I think he's a particularly generous teammate, very giving. I'm looking forward to playing a lot of cricket with him in the future."
While Warner spent Wednesday as a 12th man, carrying drinks for his Australia teammates, Root trained with England at The Oval ahead of the team's match against Sri Lanka on Thursday.
"Joe seems fine, there are no physical injuries," England captain Alastair Cook said. "It's unfortunate this has happened and he wants to put it behind him.
"We are clearly disappointed that the incident has happened," added Cook, who wasn't present on the night out, "but after investigating it from our side, we don't believe we have done anything wrong."
Cook said he had no problem with Root being out on a night out, saying: "When the schedule allows ... it's good to let your hair down."
Reports across the British media said Root and two other England players were in the Australian-themed Walkabout bar in central Birmingham when he was punched on the chin by Warner. The manager of the bar told Sky Sports on Wednesday that the players made up after the altercation and Warner picked up the bar tab.
Bailey used an unfortunate turn of phrase to describe Warner's emotional state after learning he had been dropped.
"I thought he took it on the chin and his attitude around the group was outstanding — no pun intended," Bailey said, before laughing at himself.
The temperamental Warner found himself in trouble last month when he delivered an expletive-laden Twitter rant against two senior cricket journalists.
He was fined Australian $5,750 for breaching CA's code of conduct and apologized, despite saying he disagreed with having his photo printed alongside a story written by one of the journalists about corruption in the Indian Premier League.
The potential absence of Warner, recently touted as a potential future captain, would further weaken the team's batting lineup ahead of the eagerly anticipated Ashes. Clarke missed both of Australia's matches in the Champions Trophy because of a lower back injury that could hinder him in the upcoming test series.
Warner has been a regular in Australia's test team — averaging nearly 40 — since making his debut in December 2011 but has struggled for form since arriving in England, failing to score a run in both of the warm-up games for the Champions Trophy and then 9 in the 48-run loss to England at the weekend.
During that match, Australia wicketkeeper Matthew Wade faced up to England batsman Jonathan Trott after clashing at the crease. Words were exchanged and the umpires needed to intervene.
Trott then said something to Wade as the Australian returned to the pavilion after being dismissed in the tourists' reply.
The first test starts at Trent Bridge on July 10, with the teams playing four more tests in England and then meeting in a return Ashes series in Australia. In all, the old foes meet 26 times across all formats over the next nine months.