Lancashire bowler Mahmood last playedfor his county in April and complained of back pain, with scans revealing afracture.
England return to action next month whenworld Test champions New Zealand begin a three-match tour at Lord"s on June 2but fellow fast bowlers Jofra Archer and Mark Wood are also sidelined as theycontinue to recover from elbow surgeries.
"No timeframe has been set for his(Mahmood"s) return. His ongoing rehabilitation will be co-managed betweenLancashire and England medical teams," the ECB said in a statement onMonday.
The 25-year-old quick, capable ofbowling at 90 miles per hour and generating reverse swing with the old ball,made his England Test debut earlier this year in the Caribbean.
Without the experienced James Andersonand Stuart Broad, who were dropped for the tour, Mahmood"s debut was the onebright spot in the bowling attack as England lost the three-Test series.
