diff --git a/lib/iris/fileformats/pp.py b/lib/iris/fileformats/pp.py
index 5b5f7c8a36..1c1d281871 100644
--- a/lib/iris/fileformats/pp.py
+++ b/lib/iris/fileformats/pp.py
@@ -719,8 +719,10 @@ def _data_bytes_to_shaped_array(data_bytes, lbpack, boundary_packing,
         raise iris.exceptions.NotYetImplementedError(
             'PP fields with LBPACK of %s are not yet supported.' % lbpack)
 
-    # Ensure we have write permission on the data buffer.
-    data.setflags(write=True)
+    # Ensure we have a writeable data buffer.
+    # NOTE: "data.setflags(write=True)" is not valid for numpy >= 1.16.0.
+    if not data.flags['WRITEABLE']:
+        data = data.copy()
 
     # Ensure the data is in the native byte order
     if not data.dtype.isnative:
diff --git a/requirements/core.txt b/requirements/core.txt
index 429f6573f7..7e55bd1fd5 100644
--- a/requirements/core.txt
+++ b/requirements/core.txt
@@ -6,7 +6,7 @@
 cartopy
 #conda: proj4<5
 cf_units>=2
-cftime!=1.0.2.1
+cftime==1.0.1
 dask[array]  #conda: dask
 matplotlib>=2,<3
 netcdf4