diff --git a/start b/start
index 5f9b5ca9d..3441f1f00 100755
--- a/start
+++ b/start
@@ -58,6 +58,9 @@ service_runs_migrations() {
 }
 
 NGINX_PID_FILE=/run/nginx.pid
+CUSTOM_CA_CERTS_DIR=/app/data/ssl/ca-certificates
+SYSTEM_CA_BUNDLE=/etc/ssl/certs/ca-certificates.crt
+CUSTOM_CA_BUNDLE=/tmp/custom-ca-certificates.crt
 
 fail_dep() {
     >&2 echo "$1 not running!"
@@ -274,6 +277,46 @@ ensure_saml_certificate() {
     fi
 }
 
+install_custom_ca_certificates() {
+    # Build a combined CA bundle (system certificates plus user-provided
+    # ones from the data volume) in a runtime-writable location, then point
+    # Python (requests), OpenSSL, libcurl and git at it. The system trust
+    # store is not writable for the container's unprivileged user, and the
+    # bundle is rebuilt from scratch on every start, so stale certificates
+    # never linger.
+    local certs=()
+    local cert
+    if [[ -d $CUSTOM_CA_CERTS_DIR ]]; then
+        shopt -s nullglob
+        certs=("$CUSTOM_CA_CERTS_DIR"/*.crt)
+        shopt -u nullglob
+    fi
+    if ((${#certs[@]} == 0)); then
+        return 0
+    fi
+    echo "Adding ${#certs[@]} custom CA certificate(s) from $CUSTOM_CA_CERTS_DIR to the runtime CA bundle..."
+    # Separate inputs with newlines: a source file without a trailing newline
+    # would otherwise glue PEM blocks together (e.g. "-----END CERTIFICATE----------BEGIN CERTIFICATE-----"),
+    # producing a bundle that OpenSSL/Python reject.
+    {
+        cat "$SYSTEM_CA_BUNDLE"
+        printf '\n'
+        for cert in "${certs[@]}"; do
+            cat "$cert"
+            printf '\n'
+        done
+    } > "$CUSTOM_CA_BUNDLE"
+    chmod 0644 "$CUSTOM_CA_BUNDLE"
+    export REQUESTS_CA_BUNDLE="$CUSTOM_CA_BUNDLE"
+    export SSL_CERT_FILE="$CUSTOM_CA_BUNDLE"
+    export CURL_CA_BUNDLE="$CUSTOM_CA_BUNDLE"
+    export GIT_SSL_CAINFO="$CUSTOM_CA_BUNDLE"
+    # libpq (psql) ignores SSL_CERT_FILE; without PGSSLROOTCERT the
+    # check_postgres_version probe rejects servers signed by a custom CA
+    # when POSTGRES_SSL_MODE=verify-ca/verify-full.
+    export PGSSLROOTCERT="$CUSTOM_CA_BUNDLE"
+}
+
 prepare_runtime_files() {
     prepare_ssh_keys
     ensure_data_volume_writable
@@ -283,6 +326,7 @@ prepare_runtime_files() {
         wait_for_django_secret
     fi
     ensure_saml_certificate
+    install_custom_ca_certificates
 }
 
 ensure_runtime_user() {
diff --git a/tests/run b/tests/run
index a1fced04a..a51a07a0b 100755
--- a/tests/run
+++ b/tests/run
@@ -14,3 +14,6 @@ echo 'Running Docker build arguments tests'
 
 echo 'Running image signing tests'
 ./tests/test-sign-image-tags
+
+echo 'Running custom CA certificate tests'
+./tests/test-custom-ca-certificates
diff --git a/tests/test-custom-ca-certificates b/tests/test-custom-ca-certificates
new file mode 100755
index 000000000..f70b3cd54
--- /dev/null
+++ b/tests/test-custom-ca-certificates
@@ -0,0 +1,85 @@
+#!/bin/bash
+# Tests for install_custom_ca_certificates in the start script: the function
+# is extracted verbatim and exercised against stub directories.
+
+set -eu
+
+test_dir=$(mktemp -d)
+trap 'rm -rf "$test_dir"' EXIT HUP INT TERM
+
+# Extract the path variables and the function itself from start.
+awk '/^CUSTOM_CA_CERTS_DIR=|^SYSTEM_CA_BUNDLE=|^CUSTOM_CA_BUNDLE=/' start > "$test_dir/vars.sh"
+awk '/^install_custom_ca_certificates\(\) \{/,/^\}/' start > "$test_dir/function.sh"
+grep -q 'GIT_SSL_CAINFO' "$test_dir/function.sh"
+grep -q 'PGSSLROOTCERT' "$test_dir/function.sh"
+
+# Stub "system" bundle used as the base of the combined bundle.
+printf 'system-ca\n' > "$test_dir/system.crt"
+
+# Evaluate vars + function in a subshell per case. env -u strips inherited
+# CA variables so a proxy or custom CA configured on the host cannot leak
+# into the assertions.
+run_case() {
+    env -u REQUESTS_CA_BUNDLE -u SSL_CERT_FILE -u CURL_CA_BUNDLE -u GIT_SSL_CAINFO -u PGSSLROOTCERT bash -c "
+        set -eu
+        . '$test_dir/vars.sh'
+        . '$test_dir/function.sh'
+        CUSTOM_CA_CERTS_DIR='$1'
+        SYSTEM_CA_BUNDLE='$test_dir/system.crt'
+        CUSTOM_CA_BUNDLE='$test_dir/bundle.crt'
+        install_custom_ca_certificates
+        echo \"exports: \${REQUESTS_CA_BUNDLE:-unset} \${SSL_CERT_FILE:-unset} \${CURL_CA_BUNDLE:-unset} \${GIT_SSL_CAINFO:-unset} \${PGSSLROOTCERT:-unset}\"
+    " | tail -n 1
+}
+
+fail=0
+check() { # $1 = label, $2 = expected, $3 = actual
+    if [ "$2" = "$3" ]; then
+        echo "ok: $1"
+    else
+        echo "FAIL: $1 (expected [$2], got [$3])"
+        fail=1
+    fi
+}
+
+# Case 1: no source directory -> no-op, no exports, no bundle.
+rm -f "$test_dir/bundle.crt"
+out=$(run_case "$test_dir/no-such-dir")
+check 'no dir: no exports' 'exports: unset unset unset unset unset' "$out"
+check 'no dir: no bundle written' 'missing' "$(cat "$test_dir/bundle.crt" 2> /dev/null || echo missing)"
+
+# Case 2: two certificates -> combined bundle written, exports set.
+mkdir -p "$test_dir/src2"
+printf 'cert-a\n' > "$test_dir/src2/internal-a.crt"
+printf 'cert-b\n' > "$test_dir/src2/internal-b.crt"
+touch "$test_dir/src2/ignored.pem"
+rm -f "$test_dir/bundle.crt"
+out=$(run_case "$test_dir/src2")
+check 'two certs: exports point at bundle' "exports: $test_dir/bundle.crt $test_dir/bundle.crt $test_dir/bundle.crt $test_dir/bundle.crt $test_dir/bundle.crt" "$out"
+check 'two certs: bundle = system + custom' 'system-ca cert-a cert-b' "$(tr '\n' ' ' < "$test_dir/bundle.crt" | tr -s ' ' | sed 's/ $//')"
+check 'two certs: bundle is world-readable' '644' "$(stat -c '%a' "$test_dir/bundle.crt")"
+
+# Case 4: a certificate without a trailing newline must not glue PEM blocks.
+mkdir -p "$test_dir/src4"
+printf 'cert-no-newline' > "$test_dir/src4/internal-a.crt"
+printf 'cert-after\n' > "$test_dir/src4/internal-b.crt"
+rm -f "$test_dir/bundle.crt"
+run_case "$test_dir/src4" > /dev/null
+check 'no trailing newline: first cert on its own line' '1' "$(
+    grep -xc 'cert-no-newline' "$test_dir/bundle.crt"
+    true
+)"
+check 'no trailing newline: second cert on its own line' '1' "$(
+    grep -xc 'cert-after' "$test_dir/bundle.crt"
+    true
+)"
+check 'no trailing newline: no glued content' '0' "$(grep -c 'cert-no-newlinecert-after' "$test_dir/bundle.crt" || true)"
+
+# Case 3: empty source directory -> no-op.
+mkdir -p "$test_dir/src3"
+rm -f "$test_dir/bundle.crt"
+out=$(run_case "$test_dir/src3")
+check 'empty dir: no exports' 'exports: unset unset unset unset unset' "$out"
+check 'empty dir: no bundle written' 'missing' "$(cat "$test_dir/bundle.crt" 2> /dev/null || echo missing)"
+
+exit "$fail"