England spinner Leach ruled out of Ashes with back stress fracture
The 31-year-old developed the injury as England wrapped up victory in a lone Test against Ireland on Saturday.
"A scan on Sunday in London has revealed a stress fracture, which will keep him out of the forthcoming Ashes Test series," the England and Wales Cricket Board said in a statement.
No replacement for the Somerset left-armer has been announced, but Leach's absence leaves a big gap to fill for the selectors for the showdown against Australia.
He was the only spinner named in the 16-man squad on Saturday for the first two Tests and England's depth in that area will now be tested.
Leach impressed during the winter tours of Pakistan and New Zealand to improve his Test tally to 124 wickets in 35 matches.
But teenager Rehan Ahmed also shone on his Test debut in Pakistan, taking seven wickets in one match at an average of less than 20 runs.
Surrey all-rounder Will Jacks is also in contention after he started the first two Tests in Pakistan.
England head coach Brendon McCullum and captain Ben Stokes could opt for an all-seam attack, with Joe Root providing a spin option.
But that could be a risky approach with doubts over how many overs Stokes will be fit to bowl due to a knee injury.
Root, meanwhile, is relishing the chance to focus on his batting after three Ashes as captain.
"It's going to be nice to just go and play the game and try and assist where I can for Ben and for the other guys around, play a slightly different senior role in the team," said Root.
"Obviously (I'll) try and help out where I can but Ben knows what he's doing.
"He's got enough behind him now to give himself the encouragement and confidence to make those big calls on his own, as you'd expect him to anyway. He's doing a far better job than I did."
Root's own preparation for the series had been far from ideal as he skipped County Championship cricket in April and May to take up a lucrative Indian Premier League (IPL) contract with the Rajasthan Royals.
But he was only selected three times and batted just once in the tournament.
The 32-year-old, though, believes it was still more beneficial for him to gain the experience of the IPL than more red-ball practice.
"I am not saying it's not important or a good standard. For where I am in my career, am I going to learn more about myself in that environment?" Root said.
"By learning and experiencing something new, talking and discussing the game with greats like Kumar Sangakkara and Brian Lara, other players and ex-players about just batting in general, Test cricket, I thought that not just for the Ashes, but the rest of the year for me it would set me up best to perform well and get the best out of myself. I feel ready."
The first of the five-Test Ashes series gets underway on June 16th at Edgbaston.
