diff --git a/docs/changelog/1140.bugfix.rst b/docs/changelog/1140.bugfix.rst
new file mode 100644
index 00000000..821a0d9f
--- /dev/null
+++ b/docs/changelog/1140.bugfix.rst
@@ -0,0 +1,4 @@
+Pass ``--dependency-constraints-txt`` files through to the installer unmodified instead of re-parsing them into a
+deduplicated set of lines, fixing a case where a hashed requirement (e.g. from ``pip-compile --generate-hashes``) could
+have its ``--hash`` continuation line separated from its requirement line and silently dropped, depending on the
+interpreter's hash seed - by :user:`manfred-kaiser`
diff --git a/src/build/__main__.py b/src/build/__main__.py
index fdcc9517..37e5afa0 100644
--- a/src/build/__main__.py
+++ b/src/build/__main__.py
@@ -224,7 +224,14 @@ def _bootstrap_build_env(
             install = env.install
             if dependency_constraints_txt:
                 with open(dependency_constraints_txt, encoding='utf-8') as dependency_constraints_file:
-                    install = partial(install, constraints=set(map(str.strip, dependency_constraints_file)))
+                    constraints_text = dependency_constraints_file.read()
+                if constraints_text.strip():
+                    # Passed through as a single element instead of re-parsed into lines, so a requirement can never
+                    # be split from its `--hash` continuation lines (as produced by `pip-compile --generate-hashes`)
+                    # and silently lose its hash check. env.py's installer backends reconstruct the original file via
+                    # `'\n'.join(constraints)` (see `_PipInstaller`/`_UvInstaller.install_dependencies`), a no-op here
+                    # since there is only one element.
+                    install = partial(install, constraints=(constraints_text,))
 
             # first install the build dependencies
             install(builder.build_system_requires, _fresh=True)
diff --git a/tests/test_main.py b/tests/test_main.py
index 75aba559..348e196d 100644
--- a/tests/test_main.py
+++ b/tests/test_main.py
@@ -457,7 +457,44 @@ def test_build_package_with_constraints(
     with pytest.raises(build.BuildBackendException, match=re.escape("Backend 'flit_core.buildapi' is not available.")):
         build.__main__.build_package(package_test_flit, tmp_path, ['wheel'], dependency_constraints_txt=constraints_txt_path)
 
-    install.assert_any_call({'flit_core >=2,<4'}, constraints={'flit-core==12.34', 'foo==wot'}, _fresh=True)
+    install.assert_any_call({'flit_core >=2,<4'}, constraints=('flit-core==12.34\nfoo==wot\n',), _fresh=True)
+
+
+@pytest.mark.isolated
+def test_build_package_with_constraints_passes_file_through_unmodified(
+    mocker: pytest_mock.MockerFixture, tmp_path: pathlib.Path, package_test_flit: str
+) -> None:
+    # As produced by e.g. `pip-compile --generate-hashes`: a requirement and its --hash options wrapped onto
+    # continuation lines. Regression test for the requirement/hash pair being split apart when re-parsed into
+    # individual lines - the file content must reach the installer byte-for-byte instead.
+    install = mocker.patch('build.env.DefaultIsolatedEnv.install')
+
+    constraints_text = (
+        'flit-core==12.34 \\\n    --hash=sha256:aaaa \\\n    --hash=sha256:bbbb\n    # via test\nfoo==wot \\\n'
+        '    --hash=sha256:cccc\n'
+    )
+    constraints_txt_path = tmp_path.joinpath('constraints.txt')
+    constraints_txt_path.write_text(constraints_text, encoding='utf-8')
+
+    with pytest.raises(build.BuildBackendException, match=re.escape("Backend 'flit_core.buildapi' is not available.")):
+        build.__main__.build_package(package_test_flit, tmp_path, ['wheel'], dependency_constraints_txt=constraints_txt_path)
+
+    install.assert_any_call({'flit_core >=2,<4'}, constraints=(constraints_text,), _fresh=True)
+
+
+@pytest.mark.isolated
+def test_build_package_with_empty_constraints_txt(
+    mocker: pytest_mock.MockerFixture, tmp_path: pathlib.Path, package_test_flit: str
+) -> None:
+    install = mocker.patch('build.env.DefaultIsolatedEnv.install')
+
+    constraints_txt_path = tmp_path.joinpath('constraints.txt')
+    constraints_txt_path.write_text('  \n\n', encoding='utf-8')
+
+    with pytest.raises(build.BuildBackendException, match=re.escape("Backend 'flit_core.buildapi' is not available.")):
+        build.__main__.build_package(package_test_flit, tmp_path, ['wheel'], dependency_constraints_txt=constraints_txt_path)
+
+    install.assert_any_call({'flit_core >=2,<4'}, _fresh=True)
 
 
 @pytest.mark.pypy3323bug