diff --git a/boot/messages.pl b/boot/messages.pl
index 134a470469..acd26c32c2 100644
--- a/boot/messages.pl
+++ b/boot/messages.pl
@@ -786,6 +786,8 @@
     ].
 prolog_message(untable(PI)) -->
     [ 'Reconsult: removed tabling for ~p'-[PI] ].
+prolog_message(unknown_option(Set, Opt)) -->
+    [ 'Unknown ~w option: ~p'-[Set, Opt] ].
 
 
                  /*******************************
diff --git a/man/overview.doc b/man/overview.doc
index 5e308e28b8..761649e547 100644
--- a/man/overview.doc
+++ b/man/overview.doc
@@ -2442,6 +2442,19 @@ application modules or for a specific module.  It is strongly advised
 to keep the \const{error} default and use dynamic/1 and/or multifile/1
 to specify possible non-existence of a predicate.
 
+    \prologflagitem{unknown_option}{ignore,warning,error}{rw}
+Determines the behaviour if a predicate that processes an option list
+is passed an option that is not understood by the predicate.  The ISO
+standard dictates raising a \const{domain_error} exception.  This is
+considered impractical as it makes it hard to write portable code if
+different Prolog systems support different options and it makes it
+hard to write predicates that process options and pass some of the
+options to one predicate and others to some other predicate.  For
+example, a predicate reading a file to a list of terms must distribute
+options to open/4 and read_term/3.  SWI-Prolog has always ignored
+unknown options unless in ISO mode (see the \prologflag{iso} flag).
+This flag provides full control over how options are processed.
+
     \prologflagitem{unload_foreign_libraries}{bool}{rw}
 If \const{true} (default \const{false}), unload all loaded foreign
 libraries.  Default is \const{false} because modern OSes reclaim the
diff --git a/src/ATOMS b/src/ATOMS
index 89884f1a09..bec5b8f92e 100644
--- a/src/ATOMS
+++ b/src/ATOMS
@@ -935,6 +935,7 @@ A uninstantiation_error	"uninstantiation_error"
 A unique		"unique"
 A univ			"=.."
 A unknown		"unknown"
+A unknown_option	"unknown_option"
 A unlimited		"unlimited"
 A unload		"unload"
 A unlock		"unlock"
@@ -1308,6 +1309,7 @@ F type_error		2
 F undefinterc		4
 F unify_determined	2
 F uninstantiation_error	1
+F unknown_option	2
 F unwind		1
 F var			1
 F waiting		1
diff --git a/src/SWI-Prolog.h b/src/SWI-Prolog.h
index 1cdbddee41..1daa688a59 100644
--- a/src/SWI-Prolog.h
+++ b/src/SWI-Prolog.h
@@ -1178,7 +1178,12 @@ typedef enum
 #define OPT_TYPE_MASK	0xff
 #define OPT_INF		0x100		/* allow 'inf' */
 
-#define OPT_ALL		0x1		/* flags */
+#define OPT_UNKNOWN_DEFAULT 0x0		/* Default (from flag) */
+#define OPT_UNKNOWN_ERROR   0x1		/* Unknown Prolog flags raise error */
+#define OPT_UNKNOWN_IGNORE  0x2		/* Unknown Prolog flags are ignored */
+#define OPT_UNKNOWN_WARNING 0x3		/* Unknown Prolog flags warn */
+#define OPT_UNKNOWN_MASK    0x3
+#define OPT_ALL OPT_UNKNOWN_ERROR	/* Compatibility; deprecated */
 
 typedef struct
 { atom_t		name;		/* Name of option */
diff --git a/src/Tests/core/test_scan_options.pl b/src/Tests/core/test_scan_options.pl
index 9009486f99..6dad9fb6e2 100644
--- a/src/Tests/core/test_scan_options.pl
+++ b/src/Tests/core/test_scan_options.pl
@@ -61,8 +61,6 @@
 	numbervars(x(_,X,X), 0, End, [unlikely(true)]).
 test(bad_value_type, error(type_error(bool, 42))) :-
 	numbervars(x(_,X,X), 0, _, [singletons(42)]).
-test(bad_type, error(type_error(option, unlikely))) :-
-	numbervars(x(_,X,X), 0, _, [unlikely]).
 test(bad_type, error(type_error(option, f(a,b)))) :-
 	numbervars(x(_,X,X), 0, _, [f(a,b)]).
 test(bad_type, error(type_error(option, 1.3))) :-
diff --git a/src/os/pl-option.c b/src/os/pl-option.c
index d7ea14f35d..da05a61f1f 100644
--- a/src/os/pl-option.c
+++ b/src/os/pl-option.c
@@ -3,7 +3,7 @@
     Author:        Jan Wielemaker
     E-mail:        J.Wielemaker@vu.nl
     WWW:           http://www.swi-prolog.org
-    Copyright (c)  2011-2022, University of Amsterdam
+    Copyright (c)  2011-2024, University of Amsterdam
 			      VU University Amsterdam
 			      SWI-Prolog Solutions b.v.
     All rights reserved.
@@ -43,6 +43,7 @@ Option list (or dict) processing.  See PL_scan_options() for details.
 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
 
 #define MAXOPTIONS 64
+#define HAS_OPT_MODE(f,m) (((f)&OPT_UNKNOWN_MASK) == (m))
 
 typedef union
 { int      *b;				/* boolean value */
@@ -157,7 +158,8 @@ typedef struct dictopt_ctx
   int			flags;
 } dictopt_ctx;
 
-#define dict_option(key, value, last, closure) LDFUNC(dict_option, key, value, last, closure)
+#define dict_option(key, value, last, closure) \
+	LDFUNC(dict_option, key, value, last, closure)
 
 static int
 dict_option(DECL_LD term_t key, term_t value, int last, void *closure)
@@ -177,14 +179,23 @@ dict_option(DECL_LD term_t key, term_t value, int last, void *closure)
     }
   }
 
-  if ( (ctx->flags&OPT_ALL) )
+  if ( !HAS_OPT_MODE(ctx->flags, OPT_UNKNOWN_IGNORE) )
   { term_t kv;
-    int rc = ( (kv=PL_new_term_ref()) &&
-	       PL_cons_functor(kv, FUNCTOR_colon2, key, value) &&
-	       PL_domain_error(ctx->opttype, kv)
-	     );
-    (void)rc;
-    return -1;
+
+    if ( !((kv=PL_new_term_ref()) &&
+	   PL_cons_functor(kv, FUNCTOR_colon2, key, value)) )
+      return -1;
+
+    if ( HAS_OPT_MODE(ctx->flags, OPT_UNKNOWN_ERROR) )
+    { if ( !PL_domain_error(ctx->opttype, kv) )
+	return -1;
+      return -1;
+    }
+    if ( !printMessage(ATOM_warning,
+			 PL_FUNCTOR, FUNCTOR_unknown_option2,
+			   PL_CHARS, ctx->opttype,
+			   PL_TERM, kv) )
+      return -1;
   }
 
   return 0;				/* unprocessed key */
@@ -217,7 +228,7 @@ dict_options(DECL_LD term_t dict, int flags, const char *opttype,
   ctx.flags   = flags;
   ctx.opttype = opttype;
 
-  return _PL_for_dict(dict, dict_option, &ctx, 0) == 0 ? true : false;
+  return _PL_for_dict(dict, dict_option, &ctx, 0) == 0;
 }
 
 #define vscan_options(list, flags, name, specs, args) \
@@ -235,8 +246,8 @@ vscan_options(DECL_LD term_t options, int flags, const char *opttype,
   int count = 0;
   (void)opttype;
 
-  if ( truePrologFlag(PLFLAG_ISO) )
-    flags |= OPT_ALL;
+  if ( flags == OPT_UNKNOWN_DEFAULT )
+    flags = LD->prolog_flag.unknown_option;
 
   for( n=0, s = specs; s->name; s++, n++ )
   { if ( n >= MAXOPTIONS )
@@ -272,13 +283,11 @@ vscan_options(DECL_LD term_t options, int flags, const char *opttype,
 	_PL_get_arg(2, head, val);
       } else if ( arity == 1 )
       { _PL_get_arg(1, head, val);
-      } else if ( arity == 0 )
+      } else if ( arity == 0 && !truePrologFlag(PLFLAG_ISO) )
       { implicit_true = true;
       } else
       { goto itemerror;
       }
-    } else if ( PL_is_variable(head) )
-    { return PL_error(NULL, 0, NULL, ERR_INSTANTIATION);
     } else
     { itemerror:
       return PL_error(NULL, 0, NULL, ERR_TYPE, ATOM_option, head);
@@ -301,10 +310,14 @@ vscan_options(DECL_LD term_t options, int flags, const char *opttype,
       }
     }
 
-    if ( !s->name && (implicit_true || (flags & OPT_ALL)) )
-    { if ( implicit_true )
-	goto itemerror;
-      return PL_domain_error(opttype, head);
+    if ( !s->name && !HAS_OPT_MODE(flags, OPT_UNKNOWN_IGNORE) )
+    { if ( HAS_OPT_MODE(flags, OPT_UNKNOWN_ERROR) )
+	return PL_domain_error(opttype, head);
+      if ( !printMessage(ATOM_warning,
+			 PL_FUNCTOR, FUNCTOR_unknown_option2,
+			   PL_CHARS, opttype,
+			   PL_TERM, head) )
+	return false;
     }
   }
 
diff --git a/src/os/pl-prologflag.c b/src/os/pl-prologflag.c
index d24bdf1e5a..ee09038c29 100644
--- a/src/os/pl-prologflag.c
+++ b/src/os/pl-prologflag.c
@@ -487,7 +487,7 @@ setRationalSyntax(atom_t a, unsigned int *flagp)
 
 
 
-static int
+static bool
 setUnknown(term_t value, atom_t a, Module m)
 { unsigned int flags = m->flags & ~(UNKNOWN_MASK);
 
@@ -523,6 +523,22 @@ setUnknown(term_t value, atom_t a, Module m)
 }
 
 
+static bool
+setUnknownOption(term_t value, atom_t a)
+{ GET_LD
+
+  if ( a == ATOM_ignore )
+  { LD->prolog_flag.unknown_option = OPT_UNKNOWN_IGNORE;
+  } else if ( a == ATOM_warning )
+  { LD->prolog_flag.unknown_option = OPT_UNKNOWN_WARNING;
+  } else if ( a == ATOM_error )
+  { LD->prolog_flag.unknown_option = OPT_UNKNOWN_ERROR;
+  } else
+    return PL_domain_error("unknown_option", value);
+
+  return true;
+}
+
 static int
 checkOnError(term_t value, atom_t a, atom_t key)
 { if ( a == ATOM_print || a == ATOM_halt || a == ATOM_status )
@@ -1123,6 +1139,8 @@ set_prolog_flag_unlocked(DECL_LD Module m, atom_t k, term_t value, unsigned shor
       { rval = setRationalSyntax(a, &m->flags);
       } else if ( k == ATOM_unknown )
       { rval = setUnknown(value, a, m);
+      } else if ( k == ATOM_unknown_option )
+      { rval = setUnknownOption(value, a);
       } else if ( k == ATOM_on_error || k == ATOM_on_warning )
       { rval = checkOnError(value, a, k);
       } else if ( k == ATOM_write_attributes )
@@ -1980,6 +1998,8 @@ initPrologFlags(void)
   setPrologFlag("portable_vmi", FT_BOOL, true, PLFLAG_PORTABLE_VMI);
   setPrologFlag("traditional", FT_BOOL|FF_READONLY, GD->options.traditional, 0);
   setPrologFlag("unknown", FT_ATOM, "error");
+  setPrologFlag("unknown_option", FT_ATOM, "ignore");
+  LD->prolog_flag.unknown_option = OPT_UNKNOWN_IGNORE;
   setPrologFlag("debug", FT_BOOL, false, 0);
   setPrologFlag("debug_on_interrupt", FT_BOOL,
 		truePrologFlag(PLFLAG_DEBUG_ON_INTERRUPT),
diff --git a/src/pl-global.h b/src/pl-global.h
index 48c1c51996..51ad58efbf 100644
--- a/src/pl-global.h
+++ b/src/pl-global.h
@@ -709,6 +709,7 @@ struct PL_local_data
     int		  write_attributes;	/* how to write attvars? */
     occurs_check_t occurs_check;	/* Unify and occurs check */
     access_level_t access_level;	/* Current access level */
+    unsigned int   unknown_option;	/* OPT_UNKNOWN_* */
   } prolog_flag;
 
   struct
diff --git a/src/pl-thread.c b/src/pl-thread.c
index 76225d2191..663fc20211 100644
--- a/src/pl-thread.c
+++ b/src/pl-thread.c
@@ -2199,6 +2199,7 @@ copy_local_data(PL_local_data_t *ldnew, PL_local_data_t *ldold,
   ldnew->prolog_flag.mask	  = ldold->prolog_flag.mask;
   ldnew->prolog_flag.occurs_check = ldold->prolog_flag.occurs_check;
   ldnew->prolog_flag.access_level = ldold->prolog_flag.access_level;
+  ldnew->prolog_flag.unknown_option = ldold->prolog_flag.unknown_option;
 #ifdef O_BIGNUM
   ldnew->arith.rat                = ldold->arith.rat;
 #endif