diff --git a/CHANGES.rst b/CHANGES.rst
index 96a89339c..44cfd6dcd 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -20,6 +20,7 @@ fixes:
 - chore: various minor formatting improvements (#1541)
 - docs: update spark plugin reference (#1546)
 - fix: python 2 version references in docs and init template (#1543)
+- backends: deprecate built-in Slack and SlackRTM (#1526)
 
 v6.1.8 (2021-06-21)
 -------------------
diff --git a/errbot/backends/slack.py b/errbot/backends/slack.py
index 3ae19775f..a9c459326 100644
--- a/errbot/backends/slack.py
+++ b/errbot/backends/slack.py
@@ -415,6 +415,12 @@ def update_alternate_prefixes(self):
         log.debug("Converted bot_alt_prefixes: %s", self.bot_config.BOT_ALT_PREFIXES)
 
     def serve_once(self):
+        log.warning(
+            "This backend is deprecated and will be removed in a future release."
+            " No future updates, bug fixes or enhancements will be included."
+            " We strongly advise migrating to SlackV3, which is available"
+            " at https://github.com/errbotio/err-backend-slackv3."
+        )
         self.sc = SlackClient(self.token, proxies=self.proxies)
 
         log.info("Verifying authentication token")
diff --git a/errbot/backends/slack_rtm.py b/errbot/backends/slack_rtm.py
index 57deb0526..1b7c4ec65 100644
--- a/errbot/backends/slack_rtm.py
+++ b/errbot/backends/slack_rtm.py
@@ -427,6 +427,12 @@ def serve_presences(**payload):
             self._presence_change_event_handler(payload["web_client"], payload["data"])
 
     def serve_forever(self):
+        log.warning(
+            "This backend is deprecated and will be removed in a future release."
+            " No future updates, bug fixes or enhancements will be included."
+            " We strongly advise migrating to SlackV3, which is available"
+            " at https://github.com/errbotio/err-backend-slackv3."
+        )
         self.sc = RTMClient(token=self.token, proxy=self.proxies)
 
         @RTMClient.run_on(event="open")