diff --git a/doc/RELEASE-NOTES.md b/doc/RELEASE-NOTES.md
index 539cb2c7e..e9d2b52fb 100644
--- a/doc/RELEASE-NOTES.md
+++ b/doc/RELEASE-NOTES.md
@@ -31,6 +31,8 @@ This is work in progress and may not always be a stable version.
   * `@define` can now use value-returning functions, e.g.,
     `@define $ADMIN_EMAIL environment("ADMIN_EMAIL")` to set a
     variable from an OS environment variable.
+  * New `@error "message"` and `@warning "message"` directives.
+    Eg for `if !environment("ADMIN")` `@error "ADMIN env var not defined"`.
 
 ### Changes:
 * [GeoIP](https://www.unrealircd.org/docs/GeoIP):
diff --git a/include/h.h b/include/h.h
index cfa4d3832..1aaad2189 100644
--- a/include/h.h
+++ b/include/h.h
@@ -80,6 +80,7 @@ extern PreprocessorItem parse_preprocessor_item(char *start, char *end, const ch
 extern void preprocessor_cc_duplicate_list(ConditionalConfig *r, ConditionalConfig **out);
 extern void preprocessor_cc_free_level(ConditionalConfig **cc_list, int level);
 extern void preprocessor_cc_free_list(ConditionalConfig *cc);
+extern int preprocessor_resolve_if(ConditionalConfig *cc, PreprocessorPhase phase);
 extern void preprocessor_resolve_conditionals_ce(ConfigEntry **ce_list, PreprocessorPhase phase);
 extern void preprocessor_resolve_conditionals_all(PreprocessorPhase phase);
 extern void init_config_defines(void);
diff --git a/include/struct.h b/include/struct.h
index e69916192..d1f29b73d 100644
--- a/include/struct.h
+++ b/include/struct.h
@@ -1611,7 +1611,9 @@ typedef enum PreprocessorItem {
 	PREPROCESSOR_DEFINE		= 1,
 	PREPROCESSOR_IF			= 2,
 	PREPROCESSOR_ENDIF		= 3,
-	PREPROCESSOR_ELSE		= 4
+	PREPROCESSOR_ELSE		= 4,
+	PREPROCESSOR_USER_ERROR		= 5,
+	PREPROCESSOR_USER_WARNING	= 6
 } PreprocessorItem;
 
 typedef enum PreprocessorPhase {
diff --git a/src/conf.c b/src/conf.c
index 6184846e7..7a2b6e054 100644
--- a/src/conf.c
+++ b/src/conf.c
@@ -1286,6 +1286,19 @@ ConfigFile *config_parse_with_offset(const char *filename, char *confdata, unsig
 						break;
 				}
 				cc = NULL;
+				/* If we are inside a false @if block, skip directives
+				 * that have side effects (@define, @error, @warning).
+				 * We must still process @if/@else/@endif for nesting.
+				 */
+				if (cc_list &&
+				    !preprocessor_resolve_if(cc_list, PREPROCESSOR_PHASE_INITIAL) &&
+				    strncmp(start, "@if ", 4) &&
+				    strncmp(start, "@else", 5) &&
+				    strncmp(start, "@endif", 6))
+				{
+					linenumber++;
+					break;
+				}
 				n = parse_preprocessor_item(start, ptr, filename, linenumber, &cc);
 				linenumber++;
 				if (n == PREPROCESSOR_IF)
@@ -1338,7 +1351,12 @@ ConfigFile *config_parse_with_offset(const char *filename, char *confdata, unsig
 				{
 					errors++;
 					goto breakout;
+				} else
+				if (n == PREPROCESSOR_USER_ERROR)
+				{
+					errors++;
 				}
+				/* PREPROCESSOR_USER_WARNING: nothing to do, message already printed */
 
 				if (!*ptr)
 					goto breakout; /* special case, since we don't want the for loop to ptr++ */
diff --git a/src/conf_preprocessor.c b/src/conf_preprocessor.c
index 88822c1ec..d8be9c119 100644
--- a/src/conf_preprocessor.c
+++ b/src/conf_preprocessor.c
@@ -476,6 +476,38 @@ PreprocessorItem  evaluate_preprocessor_define(char *statement, const char *file
 	return PREPROCESSOR_DEFINE;
 }
 
+/** Evaluate an @error or @warning directive.
+ * @param statement  Text after "@error " or "@warning "
+ * @param is_error   1 for @error, 0 for @warning
+ * @param filename   Config filename (for error messages)
+ * @param linenumber Line number (for error messages)
+ * @returns PREPROCESSOR_USER_ERROR or PREPROCESSOR_USER_WARNING
+ */
+static PreprocessorItem evaluate_preprocessor_error_or_warning(char *statement,
+	int is_error, const char *filename, int linenumber)
+{
+	char *p = statement;
+
+	skip_whitespace(&p);
+	if (*p == '"')
+	{
+		p++;
+		char *msg = p;
+		read_until(&p, "\"");
+		if (*p)
+			*p = '\0';
+		p = msg;
+	}
+
+	if (is_error)
+	{
+		config_error("%s:%i: %s", filename, linenumber, p);
+		return PREPROCESSOR_USER_ERROR;
+	}
+	config_warn("%s:%i: %s", filename, linenumber, p);
+	return PREPROCESSOR_USER_WARNING;
+}
+
 PreprocessorItem  parse_preprocessor_item(char *start, char *end, const char *filename, int linenumber, ConditionalConfig **cc)
 {
 	char buf[512];
@@ -496,6 +528,10 @@ PreprocessorItem  parse_preprocessor_item(char *start, char *end, const char *fi
 		return PREPROCESSOR_ELSE;
 	else if (!strncmp(buf, "@endif", 6))
 		return PREPROCESSOR_ENDIF;
+	else if (!strncmp(buf, "@error ", 7))
+		return evaluate_preprocessor_error_or_warning(buf+7, 1, filename, linenumber);
+	else if (!strncmp(buf, "@warning ", 9))
+		return evaluate_preprocessor_error_or_warning(buf+9, 0, filename, linenumber);
 
 	config_error("%s:%i: Unknown preprocessor directive: %s", filename, linenumber, buf);
 	return PREPROCESSOR_ERROR; /* ??? */