doc/cmds.texi     | 7 +++++++
 doc/features.texi | 1 +
 doc/news.texi     | 4 ++++
 main.go           | 1 +
 run.go            | 7 ++++++-

diff --git a/doc/cmds.texi b/doc/cmds.texi
index 543e6191a9e643d20ae7224e0921c5bcf7c39de638cd9e1fb1db0654dff3b259..1c0b27369b00421769892dece37dc6c1529edcf0b1336839ed0421fbaaeef4c8 100644
--- a/doc/cmds.texi
+++ b/doc/cmds.texi
@@ -46,6 +46,13 @@ By default all build commands use @code{fsync} to assure data is reached
 the disk. You can disable its usage with @env{$REDO_NO_SYNC=1}
 environment variable, for speeding up the build process.
 
+If redo sees some target modified externally, then by default it warns
+user about that, does not build that target, but continues the build
+process further. That is convenient in most cases: you can build your
+project with manual targets alterings, without touching possibly more
+complicated @file{.do} files. With @env{$REDO_STOP_IF_MODIFIED=1} redo
+won't continue and will exit with failure message.
+
 There are other commands that could be found in other implementations too:
 
 @table @command
diff --git a/doc/features.texi b/doc/features.texi
index 57d95b999102ffb8863d16549a40a1413476d1002cc1e71156e31fd0aad7f46f..1c96936ed15eb6a62688de2540ab365d7c7bcad20cae587324088c30566f477c 100644
--- a/doc/features.texi
+++ b/doc/features.texi
@@ -12,6 +12,7 @@     @item check that @file{$1} was not touched during .do execution
     @item check that @file{stdout} and @file{$3} are not written simultaneously
     @item check that generated target was not modified "externally" outside
       the redo, preventing its overwriting, but continuing the build
+      (optionally can stop)
     @end itemize
 @item recursive, indented and serialized logs display ability, with
     priority to the depth and bad return codes, like in @command{apenwarr/redo},
diff --git a/doc/news.texi b/doc/news.texi
index b1b08386d9dc04f690c8e02252fe7a26b2ddbf481952b73f584871ec17adf296..68a5cdab126a7e62cf240f65c59eb40450e94c82121837dffbd76a0ec1e10891 100644
--- a/doc/news.texi
+++ b/doc/news.texi
@@ -13,6 +13,10 @@     @file{.redo} directories and their dependency files, checks if
     corresponding targets has the same content but different
     @code{ctime}/@code{mtime} values and rewrites dependencies with that
     updated inode information.
+@item
+    With @env{$REDO_STOP_IF_MODIFIED=1} environment variable redo will
+    stop and fail if it meet externally modified file. By default user
+    is only warned about it, but building continues for convenience.
 @end itemize
 
 @anchor{Release 1_21_0}
diff --git a/main.go b/main.go
index 866eefbd0485af821ec75e9b9af15c5e4bf0736a5a7a2f3b5f54a0175f37468f..f10e630f778d37fae9f56f4ba05b804d66b906c2d2d2545bf8490602cab56f47 100644
--- a/main.go
+++ b/main.go
@@ -198,6 +198,7 @@ 		traced = *flagTrace
 	}
 	NoColor = os.Getenv(EnvNoColor) != ""
 	NoSync = os.Getenv(EnvNoSync) == "1"
+	StopIfMod = os.Getenv(EnvStopIfMod) == "1"
 	switch s := os.Getenv(EnvInodeTrust); s {
 	case "none":
 		InodeTrust = InodeTrustNone
diff --git a/run.go b/run.go
index eab2bd8363d1bd12d6a9a4b907f0d5c55315aa65722fe1c850148f3aa1acc5a6..bb0bab4664c53a6486ee82211658748576d5fdafd96b39dc5d410e1cb628618a 100644
--- a/run.go
+++ b/run.go
@@ -51,6 +51,7 @@ 	EnvTrace        = "REDO_TRACE"
 	EnvStderrKeep   = "REDO_LOGS"
 	EnvStderrSilent = "REDO_SILENT"
 	EnvNoSync       = "REDO_NO_SYNC"
+	EnvStopIfMod    = "REDO_STOP_IF_MODIFIED"
 
 	RedoDir      = ".redo"
 	LockSuffix   = ".lock"
@@ -65,6 +66,7 @@ 	NoSync       = false
 	StderrKeep   = false
 	StderrSilent = false
 	StderrPrefix string
+	StopIfMod    = false
 	Jobs         sync.WaitGroup
 
 	flagTrace        *bool
@@ -273,8 +275,11 @@ 		lockRelease()
 		return TgtError{tgtOrig, err}
 	}
 	if modified {
+		lockRelease()
+		if StopIfMod {
+			return fmt.Errorf("%s externally modified", tgtOrig)
+		}
 		tracef(CWarn, "%s externally modified: not redoing", tgtOrig)
-		lockRelease()
 		go func() {
 			errs <- nil
 		}()