diff --git a/scrapy/core/downloader/__init__.py b/scrapy/core/downloader/__init__.py
index 7c0ee0eec..759a63f5d 100644
--- a/scrapy/core/downloader/__init__.py
+++ b/scrapy/core/downloader/__init__.py
@@ -84,8 +84,11 @@ def _get_concurrency_delay(
     concurrency: int, spider: Spider, settings: BaseSettings
 ) -> tuple[int, float]:
     delay: float = settings.getfloat("DOWNLOAD_DELAY")
-    if hasattr(spider, "download_delay"):
+    if hasattr(spider, "download_delay"):  # pragma: no cover
+        warn_on_deprecated_spider_attribute("download_delay", "DOWNLOAD_DELAY")
         delay = spider.download_delay
+    if settings.getbool("AUTOTHROTTLE_ENABLED"):
+        delay = max(delay, settings.getfloat("AUTOTHROTTLE_START_DELAY"))
 
     if hasattr(spider, "max_concurrent_requests"):  # pragma: no cover
         warn_on_deprecated_spider_attribute(
diff --git a/scrapy/extensions/throttle.py b/scrapy/extensions/throttle.py
index 542ff1cdc..82da07133 100644
--- a/scrapy/extensions/throttle.py
+++ b/scrapy/extensions/throttle.py
@@ -5,6 +5,7 @@ from typing import TYPE_CHECKING
 
 from scrapy import Request, Spider, signals
 from scrapy.exceptions import NotConfigured
+from scrapy.utils.deprecate import warn_on_deprecated_spider_attribute
 
 if TYPE_CHECKING:
     # typing.Self requires Python 3.11
@@ -45,18 +46,20 @@ class AutoThrottle:
     def _spider_opened(self, spider: Spider) -> None:
         self.mindelay = self._min_delay(spider)
         self.maxdelay = self._max_delay(spider)
-        spider.download_delay = self._start_delay(spider)  # type: ignore[attr-defined]
 
     def _min_delay(self, spider: Spider) -> float:
-        s = self.crawler.settings
-        return getattr(spider, "download_delay", s.getfloat("DOWNLOAD_DELAY"))
+        if hasattr(spider, "download_delay"):  # pragma: no cover
+            warn_on_deprecated_spider_attribute("download_delay", "DOWNLOAD_DELAY")
+            return spider.download_delay
+        return self.crawler.settings.getfloat("DOWNLOAD_DELAY")
 
     def _max_delay(self, spider: Spider) -> float:
         return self.crawler.settings.getfloat("AUTOTHROTTLE_MAX_DELAY")
 
     def _start_delay(self, spider: Spider) -> float:
         return max(
-            self.mindelay, self.crawler.settings.getfloat("AUTOTHROTTLE_START_DELAY")
+            self.mindelay,
+            self.crawler.settings.getfloat("AUTOTHROTTLE_START_DELAY"),
         )
 
     def _response_downloaded(
diff --git a/tests/test_extension_throttle.py b/tests/test_extension_throttle.py
index 4874f284a..498e0cd42 100644
--- a/tests/test_extension_throttle.py
+++ b/tests/test_extension_throttle.py
@@ -150,7 +150,7 @@ def test_startdelay_definition(min_spider, min_setting, start_setting, expected)
     at = build_from_crawler(AutoThrottle, crawler)
     spider = _TestSpider()
     at._spider_opened(spider)
-    assert spider.download_delay == expected
+    assert at._start_delay(spider) == expected
 
 
 @pytest.mark.parametrize(