diff --git a/src/corelib/thread/qthreadpool.cpp b/src/corelib/thread/qthreadpool.cpp
index 31c3f5a256b..9ef40a52097 100644
--- a/src/corelib/thread/qthreadpool.cpp
+++ b/src/corelib/thread/qthreadpool.cpp
@@ -180,6 +180,7 @@ bool QThreadPoolPrivate::tryStart(QRunnable *task)
         // recycle an available thread
         --waitingThreads;
         enqueueTask(task);
+        runnableReady.wakeOne();
         return true;
     }
 
@@ -218,7 +219,6 @@ void QThreadPoolPrivate::enqueueTask(QRunnable *runnable, int priority)
     if (it != begin && priority > (*(it - 1)).second)
         it = std::upper_bound(begin, --it, priority);
     queue.insert(it - begin, qMakePair(runnable, priority));
-    runnableReady.wakeOne();
 }
 
 int QThreadPoolPrivate::activeThreadCount() const
@@ -456,8 +456,14 @@ void QThreadPool::start(QRunnable *runnable, int priority)
 
     Q_D(QThreadPool);
     QMutexLocker locker(&d->mutex);
-    if (!d->tryStart(runnable))
+    if (!d->tryStart(runnable)) {
         d->enqueueTask(runnable, priority);
+
+        if (d->waitingThreads > 0) {
+            --d->waitingThreads;
+            d->runnableReady.wakeOne();
+        }
+    }
 }
 
 /*!
diff --git a/tests/auto/corelib/thread/qthreadpool/tst_qthreadpool.cpp b/tests/auto/corelib/thread/qthreadpool/tst_qthreadpool.cpp
index 10a59ac3017..cdf240d7577 100644
--- a/tests/auto/corelib/thread/qthreadpool/tst_qthreadpool.cpp
+++ b/tests/auto/corelib/thread/qthreadpool/tst_qthreadpool.cpp
@@ -90,6 +90,7 @@ private slots:
     void reserveThread();
     void releaseThread_data();
     void releaseThread();
+    void reserveAndStart();
     void start();
     void tryStart();
     void tryStartPeakThreadCount();
@@ -630,6 +631,61 @@ void tst_QThreadPool::releaseThread()
     threadpool->setMaxThreadCount(savedLimit);
 }
 
+void tst_QThreadPool::reserveAndStart() // QTBUG-21051
+{
+    class WaitingTask : public QRunnable
+    {
+    public:
+        QAtomicInt count;
+        QSemaphore waitForStarted;
+
+        WaitingTask() { setAutoDelete(false); }
+
+        void run()
+        {
+            count.ref();
+            waitForStarted.release();
+        }
+    };
+
+    // Set up
+    QThreadPool *threadpool = QThreadPool::globalInstance();
+    int savedLimit = threadpool->maxThreadCount();
+    threadpool->setMaxThreadCount(1);
+    QCOMPARE(threadpool->activeThreadCount(), 0);
+
+    // reserve
+    threadpool->reserveThread();
+    QCOMPARE(threadpool->activeThreadCount(), 1);
+
+    // start a task, to get a running thread
+    WaitingTask *task = new WaitingTask;
+    threadpool->start(task);
+    QCOMPARE(threadpool->activeThreadCount(), 2);
+    task->waitForStarted.acquire();
+    QTRY_COMPARE(task->count.load(), 1);
+    QTRY_COMPARE(threadpool->activeThreadCount(), 1);
+
+    // now the thread is waiting, but tryStart() will fail since activeThreadCount() >= maxThreadCount()
+    QVERIFY(!threadpool->tryStart(task));
+    QTRY_COMPARE(threadpool->activeThreadCount(), 1);
+
+    // start() will therefore do a failing tryStart(), followed by enqueueTask()
+    // which will actually wake up the waiting thread.
+    threadpool->start(task);
+    QTRY_COMPARE(threadpool->activeThreadCount(), 2);
+    task->waitForStarted.acquire();
+    QTRY_COMPARE(task->count.load(), 2);
+    QTRY_COMPARE(threadpool->activeThreadCount(), 1);
+
+    threadpool->releaseThread();
+    QTRY_COMPARE(threadpool->activeThreadCount(), 0);
+
+    delete task;
+
+    threadpool->setMaxThreadCount(savedLimit);
+}
+
 QAtomicInt count;
 class CountingRunnable : public QRunnable
 {