diff --git a/changelog.d/2870.breaking.rst b/changelog.d/2870.breaking.rst
new file mode 100644
index 0000000000..1d17ede602
--- /dev/null
+++ b/changelog.d/2870.breaking.rst
@@ -0,0 +1 @@
+Started failing on invalid inline description with line breaks :class:`ValueError` -- by :user:`webknjaz`
diff --git a/setuptools/dist.py b/setuptools/dist.py
index 8e2111a52b..848d6b0f75 100644
--- a/setuptools/dist.py
+++ b/setuptools/dist.py
@@ -145,11 +145,11 @@ def read_pkg_file(self, file):
 
 
 def single_line(val):
-    # quick and dirty validation for description pypa/setuptools#1390
+    """Validate that the value does not have line breaks."""
+    # Ref: https://github.com/pypa/setuptools/issues/1390
     if '\n' in val:
-        # TODO after 2021-07-31: Replace with `raise ValueError("newlines not allowed")`
-        warnings.warn("newlines not allowed and will break in the future")
-        val = val.replace('\n', ' ')
+        raise ValueError('Newlines are not allowed')
+
     return val