diff --git a/cmake/findDependencies.cmake b/cmake/findDependencies.cmake
index 3ad0e366f..6f31bff00 100644
--- a/cmake/findDependencies.cmake
+++ b/cmake/findDependencies.cmake
@@ -7,18 +7,10 @@ if (BUILD_GUI)
         list(APPEND qt_components Test)
     endif()
     if (USE_QT6)
-        find_package(Qt6 COMPONENTS ${qt_components} QUIET)
-    endif()
-    if (NOT Qt6Core_FOUND)
-        find_package(Qt5 COMPONENTS ${qt_components} REQUIRED)
-        set(QT_VERSION "${Qt5Core_VERSION_STRING}")
-    else()
-        # detect faulty Qt6 installation by jurplel/install-qt-action@v2
-        if (WITH_QCHART AND NOT Qt6Charts_FOUND)
-            message(FATAL_ERROR "QtCharts not found")
-        endif()
+        find_package(Qt6 COMPONENTS ${qt_components} REQUIRED)
         set(QT_VERSION "${Qt6Core_VERSION_STRING}")
         if (NOT QT_VERSION)
+            # TODO: how to get the actual version?
             message(WARNING "'Qt6Core_VERSION_STRING' is not set - using 6.0.0 as fallback")
             set(QT_VERSION "6.0.0")
         endif()
@@ -31,6 +23,9 @@ if (BUILD_GUI)
             # https://bugreports.qt.io/browse/QTBUG-86395
             set(DISABLE_CRTDBG_MAP_ALLOC ON)
         endif()
+    else()
+        find_package(Qt5 COMPONENTS ${qt_components} REQUIRED)
+        set(QT_VERSION "${Qt5Core_VERSION_STRING}")
     endif()
 endif()
 
diff --git a/gui/gui.pro b/gui/gui.pro
index 186426dae..b3d4c9da9 100644
--- a/gui/gui.pro
+++ b/gui/gui.pro
@@ -1,4 +1,7 @@
 lessThan(QT_MAJOR_VERSION, 5): error(requires >= Qt 5 (You used: $$QT_VERSION))
+greaterThan(QT_MAJOR_VERSION, 5): error(Qt 6 is not supported via qmake - please use CMake instead)
+
+message("Building the GUI via qmake is deprecated and will be removed in a future release. Please use CMake instead.")
 
 TEMPLATE = app
 TARGET = cppcheck-gui
diff --git a/readme.md b/readme.md
index ccb6ae17d..5094fa050 100644
--- a/readme.md
+++ b/readme.md
@@ -86,6 +86,8 @@ cmake --build . --config RelWithDebInfo
 
 ### qmake
 
+NOTE: This has been deprecated and will be removed in a future version. Please use CMake instead.
+
 You can use the gui/gui.pro file to build the GUI.
 
 ```shell
diff --git a/releasenotes.txt b/releasenotes.txt
index 225e5b679..11e93c239 100644
--- a/releasenotes.txt
+++ b/releasenotes.txt
@@ -8,4 +8,8 @@ GUI:
 
 Changed interface:
 
+Deprecations:
+- The qmake build system has been deprecated and will be removed in a future version.
+
 Other:
+- "USE_QT6=On" will no longer fallback to Qt5 when Qt6 is not found.