diff --git a/lib/iris/io/s3_mounts.py b/lib/iris/io/s3_mounts.py
new file mode 100644
index 0000000000..c90e64c8c1
--- /dev/null
+++ b/lib/iris/io/s3_mounts.py
@@ -0,0 +1,164 @@
+# Copyright Iris contributors
+#
+# This file is part of Iris and is released under the BSD license.
+# See LICENSE in the root of the repository for full licensing details.
+"""Code to manage access to files stored in S3 buckets.
+
+S3 buckets are mounted by name in temporary directories.
+These mounts must persist, so that any data-proxies can re-open the mapped files.
+They are removed either at system exit, or under the control of a context manager.
+Authentication, access control, and the storage region supplied are controlled outside of
+Iris/Python, by user configuration (e.g. "aws configure").
+
+"""
+
+import atexit
+from pathlib import Path
+import shutil
+import subprocess
+from subprocess import run
+import time
+from typing import List
+
+_DO_DEBUG = True
+
+
+def _DEBUG(*args, **kwargs):
+    if _DO_DEBUG:
+        print(*args, **kwargs)
+
+
+class _S3MountsManager:
+    _N_UNMOUNT_RETRIES = 3
+    _NULL_PATH = Path("/<nonexist>")
+
+    def __init__(self, base_path: str | Path | None = None):
+        self.mounts_basepath: Path = self._NULL_PATH
+        self.mount_paths: List[Path] = []
+
+    def _ensure_basepath(self) -> Path:
+        if self.mounts_basepath is self._NULL_PATH:
+            base_path = Path("/var/tmp/__iris_s3mounts__")
+            if not base_path.exists():
+                base_path.mkdir(parents=True, exist_ok=False)
+            self.mounts_basepath = base_path
+            assert self.mounts_basepath.exists()
+        return self.mounts_basepath
+
+    def bucket_mountpath(self, bucket_name: str) -> Path:
+        self._ensure_basepath()
+        mount_path = self.mounts_basepath / bucket_name
+        if mount_path not in self.mount_paths:
+            _DEBUG(f"\nCreating S3 mount-path dir {mount_path} ...")
+            mount_path.mkdir(parents=True, exist_ok=False)
+            _DEBUG("...done.\n")
+
+            _DEBUG(f"\nMounting S3 bucket {bucket_name} ...")
+            try:
+                run(
+                    f"s3fs {bucket_name} {mount_path}",
+                    shell=True,
+                    capture_output=True,
+                    check=True,
+                )
+            except subprocess.CalledProcessError as exc:
+                print(f"Error mounting s3 bucket {bucket_name} at {mount_path}:")
+                print(exc.stderr.decode())
+                raise
+            _DEBUG("...done.\n")
+            self.mount_paths.append(mount_path)
+
+        return mount_path
+
+    def unmount_bucket(self, mount_path: Path, n_tries_done: int) -> bool:
+        """Attempt to unmount the specified S3 mount, or force if timed out."""
+        success = False
+        if n_tries_done >= self._N_UNMOUNT_RETRIES:
+            _DEBUG(
+                f"Unmount of {mount_path} out of retries - final attempt with lazy ..."
+            )
+            try:
+                run(
+                    f"umount {mount_path} -l",
+                    shell=True,
+                    capture_output=True,
+                    check=True,
+                )
+                _DEBUG("...succeeded.\n")
+                success = True
+            except subprocess.CalledProcessError as exc:
+                msg = exc.stderr.decode()
+                print(f"Unknown error in 'umount {mount_path} -l' :", msg)
+                raise
+        else:
+            try:
+                _DEBUG(
+                    f"\nUnmounting mount path {mount_path}, attempt #{n_tries_done} ..."
+                )
+                run(f"umount {mount_path}", shell=True, capture_output=True, check=True)
+                _DEBUG("...succeeded.\n")
+                success = True
+            except subprocess.CalledProcessError as exc:
+                msg = exc.stderr.decode()
+                if "busy" in msg:
+                    # This is OK. We will just pause before retrying.
+                    _DEBUG("Unmount request failed with 'busy': error=", msg)
+                else:
+                    print(f"Unknown error attempting to unmount {mount_path}:", msg)
+                    raise
+        return success
+
+    def unmount_all(self):
+        # Cleanup handler
+        unmount_tries = {name: 0 for name in self.mount_paths}
+        while unmount_tries:
+            # try once (each pass) to unmount each outstanding mount.
+            to_do = list(unmount_tries.keys())
+            for unmount in to_do:
+                n_tries = unmount_tries[unmount]
+                unmount_tries[unmount] = n_tries + 1
+                try:
+                    if self.unmount_bucket(unmount, n_tries):
+                        del unmount_tries[unmount]
+                except subprocess.CalledProcessError as exc:
+                    print("Other failure on unmount: ", exc.stderr.decode())
+                    raise
+
+            if unmount_tries:
+                # If any remain, pause before doing a batch of retries.
+                time.sleep(1.0)
+
+        # All finally gone: remove the base-path also
+        try:
+            shutil.rmtree(self.mounts_basepath)
+        finally:
+            self.mounts_basepath = self._NULL_PATH
+
+
+# Singleton object that holds all the info.
+s3_mounter = _S3MountsManager()
+
+
+# Fix so that we clean up at the exit of the main program.
+@atexit.register
+def _cleanup():
+    _DEBUG("\n\nFINAL CLEANUP.\n")
+    try:
+        s3_mounter.unmount_all()
+    except Exception:
+        pass  # ignore any exception with the cleanup handler
+
+
+# #
+# # ALSO: force cleanup if an uncaught Exception causes exit.
+# #
+# _orig_excepthook = sys.excepthook
+#
+# def _excepthook(*args):
+#     global _orig_excepthook
+#     try:
+#         _cleanup()
+#     finally:
+#         _orig_excepthook(*args)
+#
+# sys.excepthook = _excepthook
diff --git a/lib/iris/loading.py b/lib/iris/loading.py
index 29a6e437f4..d5311b036f 100644
--- a/lib/iris/loading.py
+++ b/lib/iris/loading.py
@@ -29,18 +29,35 @@ def _generate_cubes(uris, callback, constraints):
 
     for scheme, groups in itertools.groupby(uri_tuples, key=lambda x: x[0]):
         # Call each scheme handler with the appropriate URIs
-        if scheme == "file":
-            part_names = [x[1] for x in groups]
-            for cube in iris.io.load_files(part_names, callback, constraints):
+        if scheme in ("file", "s3"):
+            filepaths = []
+            for group in groups:
+                filepath = group[1]
+                if scheme == "s3":
+                    # S3-based files are loaded from a bucket mounted *as a filesystem*.
+                    # For this (a) ensure the mount and (b) adjust the filepath.
+                    from iris.io.s3_mounts import s3_mounter
+
+                    # Strip off initial "//"
+                    assert filepath.startswith("//")
+                    bucket_name, filepath = filepath[2:].split("/", maxsplit=1)
+                    bucket_mount_path = s3_mounter.bucket_mountpath(bucket_name)
+                    filepath = str(bucket_mount_path / filepath)
+                filepaths.append(filepath)
+
+            for cube in iris.io.load_files(filepaths, callback, constraints):
                 yield cube
+
         elif scheme in ["http", "https"]:
             urls = [":".join(x) for x in groups]
             for cube in iris.io.load_http(urls, callback):
                 yield cube
+
         elif scheme == "data":
             data_objects = [x[1] for x in groups]
             for cube in iris.io.load_data_objects(data_objects, callback):
                 yield cube
+
         else:
             raise ValueError("Iris cannot handle the URI scheme: %s" % scheme)