diff --git a/easybuild/toolchains/gcc.py b/easybuild/toolchains/gcc.py
index 0b6b2b96f9..5928e81624 100644
--- a/easybuild/toolchains/gcc.py
+++ b/easybuild/toolchains/gcc.py
@@ -31,7 +31,7 @@
 """
 import re
 
-from easybuild.toolchains.gcccore import GCCcore
+from easybuild.toolchains.gcccore import GCCcore, GCCCORE_BANNED_LIBRARIES
 from easybuild.tools import LooseVersion
 from easybuild.tools.toolchain.toolchain import SYSTEM_TOOLCHAIN_NAME
 
@@ -43,6 +43,16 @@ class GccToolchain(GCCcore):
     SUBTOOLCHAIN = [GCCcore.NAME, SYSTEM_TOOLCHAIN_NAME]
     OPTIONAL = False
 
+    def banned_linked_shared_libs(self):
+        """
+        List of shared libraries (names, file names, paths) which are
+        not allowed to be linked in any installed binary/library.
+        """
+        banned_libs = super().banned_linked_shared_libs()
+
+        # Restore libraries that were banned at GCCcore level but ok for GCC
+        return list(set(banned_libs) - set(GCCCORE_BANNED_LIBRARIES))
+
     def is_deprecated(self):
         """Return whether or not this toolchain is deprecated."""
         # GCC toolchains older than GCC version 8.x are deprecated since EasyBuild v4.5.0
diff --git a/easybuild/toolchains/gcccore.py b/easybuild/toolchains/gcccore.py
index f3ff61dae7..5e3f1b1bb4 100644
--- a/easybuild/toolchains/gcccore.py
+++ b/easybuild/toolchains/gcccore.py
@@ -35,6 +35,9 @@
 from easybuild.tools import LooseVersion
 from easybuild.tools.toolchain.toolchain import SYSTEM_TOOLCHAIN_NAME
 
+# At GCCcore level we do not allow OpenMP
+GCCCORE_BANNED_LIBRARIES = ['libgomp.so']
+
 
 class GCCcore(Gcc):
     """Compiler-only toolchain, including only GCC and binutils."""
@@ -46,6 +49,15 @@ class GCCcore(Gcc):
     # for old versions of some toolchains (GCC, intel) it is not part of the hierarchy and hence optional
     OPTIONAL = True
 
+    def banned_linked_shared_libs(self):
+        """
+        List of shared libraries (names, file names, paths) which are
+        not allowed to be linked in any installed binary/library.
+        """
+        banned_libs = super().banned_linked_shared_libs()
+
+        return banned_libs + GCCCORE_BANNED_LIBRARIES
+
     def is_deprecated(self):
         """Return whether or not this toolchain is deprecated."""
         # GCC toolchains older than GCC version 8.x are deprecated since EasyBuild v4.5.0
diff --git a/easybuild/toolchains/linalg/flexiblas.py b/easybuild/toolchains/linalg/flexiblas.py
index 73cb3ffe0c..219f70cff6 100644
--- a/easybuild/toolchains/linalg/flexiblas.py
+++ b/easybuild/toolchains/linalg/flexiblas.py
@@ -34,6 +34,7 @@
 
 from easybuild.tools.toolchain.linalg import LinAlg
 
+from easybuild.tools.options import build_option
 from easybuild.tools.run import run_shell_cmd
 from easybuild.tools.systemtools import get_shared_lib_ext
 
@@ -48,18 +49,20 @@ def det_flexiblas_backend_libs():
     # System-wide (config directory):
     #  OPENBLAS
     #    library = libflexiblas_openblas.so
-    res = run_shell_cmd("flexiblas list", hidden=True)
-
-    shlib_ext = get_shared_lib_ext()
-    flexiblas_lib_regex = re.compile(r'library = (?P<lib>lib.*\.%s)' % shlib_ext, re.M)
-    flexiblas_libs = flexiblas_lib_regex.findall(res.output)
-
     backend_libs = []
-    for flexiblas_lib in flexiblas_libs:
-        # assumption here is that the name of FlexiBLAS library (like 'libflexiblas_openblas.so')
-        # maps directly to name of the backend library ('libopenblas.so')
-        backend_lib = 'lib' + flexiblas_lib.replace('libflexiblas_', '')
-        backend_libs.append(backend_lib)
+    # in a unittest context the flexiblas command does not exist, but in a normal context it always does
+    if not build_option('unit_testing_mode'):
+        res = run_shell_cmd("flexiblas list", hidden=True)
+
+        shlib_ext = get_shared_lib_ext()
+        flexiblas_lib_regex = re.compile(r'library = (?P<lib>lib.*\.%s)' % shlib_ext, re.M)
+        flexiblas_libs = flexiblas_lib_regex.findall(res.output)
+
+        for flexiblas_lib in flexiblas_libs:
+            # assumption here is that the name of FlexiBLAS library (like 'libflexiblas_openblas.so')
+            # maps directly to name of the backend library ('libopenblas.so')
+            backend_lib = 'lib' + flexiblas_lib.replace('libflexiblas_', '')
+            backend_libs.append(backend_lib)
 
     return backend_libs