Fowler, 36, led the club to the Midland Football League Premier Division title last season, but hasn’t been able to agree on a new deal to stay with the Robins.
Chief operating officer Nick Hawkins said: “It's a sad day for the club losing someone of Lee's calibre but inevitably both parties have to be happy and we were not in a position to fulfil the original contract Lee was offered.
“We have tried tirelessly over the past month to find a common ground but it wasn't to be, we lose a great coach but more importantly a great person and he will be missed by myself, staff and players alike.
“I have no doubt Lee will go onto great things has he has an incredible work ethic and desire to succeed and I will follow his career closely.”
Fowler took charge in 2018 following a spell at Nuneaton Borough.
The former Wales Under 21s international represented numerous clubs during his playing career, including Coventry, Newport and Kidderminster.
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