diff --git a/docs/news.rst b/docs/news.rst
index ef3b549e7..8b1d51674 100644
--- a/docs/news.rst
+++ b/docs/news.rst
@@ -126,6 +126,15 @@ Backward-incompatible changes
     also enforced for start requests.
     (:issue:`6777`)
 
+-   Calling :func:`scrapy.utils.reactor.is_asyncio_reactor_installed` without
+    an installed reactor now raises an exception instead of installing a
+    reactor. This shouldn't affect normal Scrapy use cases, but it may affect
+    3rd-party test suites that use Scrapy internals such as
+    :class:`~scrapy.crawler.Crawler` and don't install a reactor explicitly. If
+    you are affected by this change, you most likely need to install the
+    reactor before running Scrapy code that expects it to be installed.
+    (:issue:`6732`, :issue:`6735`)
+
 -   The ``from_settings()`` method of
     :class:`~scrapy.spidermiddlewares.urllength.UrlLengthMiddleware`,
     deprecated in Scrapy 2.12.0, is removed earlier than the usual deprecation
diff --git a/scrapy/utils/reactor.py b/scrapy/utils/reactor.py
index 9c2754394..1b179f988 100644
--- a/scrapy/utils/reactor.py
+++ b/scrapy/utils/reactor.py
@@ -202,6 +202,11 @@ def is_asyncio_reactor_installed() -> bool:
     """Check whether the installed reactor is :class:`~twisted.internet.asyncioreactor.AsyncioSelectorReactor`.
 
     Raise a :exc:`RuntimeError` if no reactor is installed.
+
+    .. versionchanged:: 2.13
+       In earlier Scrapy versions this function silently installed the default
+       reactor if there was no reactor installed. Now it raises an exception to
+       prevent silent problems in this case.
     """
     if not is_reactor_installed():
         raise RuntimeError(
diff --git a/scrapy/utils/test.py b/scrapy/utils/test.py
index 2da526cd8..4a732bd72 100644
--- a/scrapy/utils/test.py
+++ b/scrapy/utils/test.py
@@ -18,7 +18,7 @@ from twisted.trial.unittest import SkipTest
 from scrapy.exceptions import ScrapyDeprecationWarning
 from scrapy.utils.boto import is_botocore_available
 from scrapy.utils.deprecate import create_deprecated_class
-from scrapy.utils.reactor import is_asyncio_reactor_installed
+from scrapy.utils.reactor import is_asyncio_reactor_installed, is_reactor_installed
 from scrapy.utils.spider import DefaultSpider
 
 if TYPE_CHECKING:
@@ -117,6 +117,11 @@ def get_reactor_settings() -> dict[str, Any]:
     settings, so tests that run the crawler in the current process may need to
     pass a correct ``"TWISTED_REACTOR"`` setting value when creating it.
     """
+    if not is_reactor_installed():
+        raise RuntimeError(
+            "get_reactor_settings() called without an installed reactor,"
+            " you may need to install a reactor explicitly when running your tests."
+        )
     settings: dict[str, Any] = {}
     if not is_asyncio_reactor_installed():
         settings["TWISTED_REACTOR"] = None