diff --git a/.ci/scripts/prepare_old_deps.sh b/.ci/scripts/prepare_old_deps.sh
index e536a9db8b..580f87bbdf 100755
--- a/.ci/scripts/prepare_old_deps.sh
+++ b/.ci/scripts/prepare_old_deps.sh
@@ -31,35 +31,6 @@ sed -i \
    -e '/systemd/d' \
    pyproject.toml
 
-# Use poetry to do the installation. This ensures that the versions are all mutually
-# compatible (as far the package metadata declares, anyway); pip's package resolver
-# is more lax.
-#
-# Rather than `poetry install --no-dev`, we drop all dev dependencies and the dev-docs
-# group from the toml file. This means we don't have to ensure compatibility between
-# old deps and dev tools.
-
-pip install toml wheel
-
-REMOVE_DEV_DEPENDENCIES="
-import toml
-with open('pyproject.toml', 'r') as f:
-    data = toml.loads(f.read())
-
-del data['tool']['poetry']['dev-dependencies']
-del data['tool']['poetry']['group']['dev-docs']
-
-with open('pyproject.toml', 'w') as f:
-    toml.dump(data, f)
-"
-python3 -c "$REMOVE_DEV_DEPENDENCIES"
-
-pip install poetry==1.3.2
-poetry lock
-
 echo "::group::Patched pyproject.toml"
 cat pyproject.toml
 echo "::endgroup::"
-echo "::group::Lockfile after patch"
-cat poetry.lock
-echo "::endgroup::"
diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index e128fd54f7..51cbeb3298 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -321,34 +321,25 @@ jobs:
         with:
           python-version: '3.7'
 
-      # Calculating the old-deps actually takes a bunch of time, so we cache the
-      # pyproject.toml / poetry.lock. We need to cache pyproject.toml as
-      # otherwise the `poetry install` step will error due to the poetry.lock
-      # file being outdated.
-      #
-      # This caches the output of `Prepare old deps`, which should generate the
-      # same `pyproject.toml` and `poetry.lock` for a given `pyproject.toml` input.
-      - uses: actions/cache@v3
-        id: cache-poetry-old-deps
-        name: Cache poetry.lock
-        with:
-          path: |
-            poetry.lock
-            pyproject.toml
-          key: poetry-old-deps2-${{ hashFiles('pyproject.toml') }}
       - name: Prepare old deps
         if: steps.cache-poetry-old-deps.outputs.cache-hit != 'true'
         run: .ci/scripts/prepare_old_deps.sh
 
-      # We only now install poetry so that `setup-python-poetry` caches the
-      # right poetry.lock's dependencies.
-      - uses: matrix-org/setup-python-poetry@v1
-        with:
-          python-version: '3.7'
-          poetry-version: "1.3.2"
-          extras: "all test"
+      # Note: we install using `pip` here, not poetry. `poetry install` ignores the
+      # build-system section (https://github.com/python-poetry/poetry/issues/6154), but
+      # we explicitly want to test that you can `pip install` using the oldest version
+      # of poetry-core and setuptools-rust.
+      - run: pip install .[all,test]
 
-      - run: poetry run trial -j6 tests
+      # We nuke the local copy, as we've installed synapse into the virtualenv
+      # (rather than use an editable install, which we no longer support). If we
+      # don't do this then python can't find the native lib.
+      - run: rm -rf synapse/
+
+      # Sanity check we can import/run Synapse
+      - run: python -m synapse.app.homeserver --help
+
+      - run: python -m twisted.trial -j6 tests
       - name: Dump logs
         # Logs are most useful when the command fails, always include them.
         if: ${{ always() }}
diff --git a/changelog.d/15566.bugfix b/changelog.d/15566.bugfix
index 07fac520f6..130342590c 100644
--- a/changelog.d/15566.bugfix
+++ b/changelog.d/15566.bugfix
@@ -1 +1 @@
-Require at least poetry-core v1.2.0.
+Require at least poetry-core v1.1.0.
diff --git a/changelog.d/15571.bugfix b/changelog.d/15571.bugfix
new file mode 100644
index 0000000000..130342590c
--- /dev/null
+++ b/changelog.d/15571.bugfix
@@ -0,0 +1 @@
+Require at least poetry-core v1.1.0.
diff --git a/pyproject.toml b/pyproject.toml
index 6bbc914af6..16b5d9bc3b 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -368,7 +368,7 @@ furo = ">=2022.12.7,<2024.0.0"
 # system changes.
 # We are happy to raise these upper bounds upon request,
 # provided we check that it's safe to do so (i.e. that CI passes).
-requires = ["poetry-core>=1.2.0,<=1.5.0", "setuptools_rust>=1.3,<=1.6.0"]
+requires = ["poetry-core>=1.1.0,<=1.5.0", "setuptools_rust>=1.3,<=1.6.0"]
 build-backend = "poetry.core.masonry.api"