diff --git a/Orderbook.h b/Orderbook.h
index a7ead67..de98e07 100644
--- a/Orderbook.h
+++ b/Orderbook.h
@@ -412,14 +412,12 @@ class Orderbook {
             ? asks_.at(price)
             : bids_.at(price);
 
-        // Swap-and-pop: move the last element into the cancelled slot, then
-        // pop the back. This is O(1) and keeps the vector contiguous.
-        // We must update the swapped order's stored index to reflect its new position.
-        if (idx != orders.size() - 1) {
-            orders[idx] = std::move(orders.back());
-            orders_.at(orders[idx]->GetOrderId()).location_ = idx;
+        // Erase in place to preserve FIFO time priority within the price level.
+        // The shifted suffix needs fresh indices in the lookup map.
+        orders.erase(orders.begin() + static_cast<std::ptrdiff_t>(idx));
+        for (std::size_t i = idx; i < orders.size(); ++i) {
+            orders_.at(orders[i]->GetOrderId()).location_ = i;
         }
-        orders.pop_back();
 
         if (side == Side::Sell) {
             if (orders.empty()) asks_.erase(price);
@@ -500,4 +498,4 @@ class Orderbook {
     void ResetMarketDataStats() { stats_.Reset(); }
     bool IsInitialized() const { return isInitialized_; }
     uint64_t GetLastSequenceNumber() const { return lastSequenceNumber_; }
-};
\ No newline at end of file
+};
diff --git a/tests.cpp b/tests.cpp
index aa44314..6784467 100644
--- a/tests.cpp
+++ b/tests.cpp
@@ -114,6 +114,21 @@ TEST(TestTimePriority_FIFO) {
     ASSERT_EQ(trades[0].GetBidTrade().orderId_, 1);
 }
 
+TEST(TestCancelPreservesFIFO) {
+    Orderbook orderbook;
+    orderbook.AddOrder(std::make_shared<Order>(OrderType::GoodTillCancel, 1, Side::Buy, 100, 10));
+    orderbook.AddOrder(std::make_shared<Order>(OrderType::GoodTillCancel, 2, Side::Buy, 100, 10));
+    orderbook.AddOrder(std::make_shared<Order>(OrderType::GoodTillCancel, 3, Side::Buy, 100, 10));
+
+    orderbook.CancelOrder(1);
+    auto trades = orderbook.AddOrder(
+        std::make_shared<Order>(OrderType::GoodTillCancel, 4, Side::Sell, 100, 10)
+    );
+
+    ASSERT_EQ(trades.size(), 1);
+    ASSERT_EQ(trades[0].GetBidTrade().orderId_, 2);
+}
+
 TEST(TestMarketOrderBuy) {
     Orderbook orderbook;
     orderbook.AddOrder(std::make_shared<Order>(OrderType::GoodTillCancel, 1, Side::Sell, 100, 10));
@@ -589,6 +604,7 @@ int main() {
     RUN_TEST(TestMultipleMatchesAtSamePrice);
     RUN_TEST(TestPricePriority);
     RUN_TEST(TestTimePriority_FIFO);
+    RUN_TEST(TestCancelPreservesFIFO);
     RUN_TEST(TestMarketOrderBuy);
     RUN_TEST(TestMarketOrderSell);
     RUN_TEST(TestMarketOrderEmptyBook);
@@ -603,7 +619,7 @@ int main() {
     RUN_TEST(TestMinNotionalValidation);
     RUN_TEST(TestMarketOrderValidation);
 
-    std::cout << "\nAll " << 21 << " functionality tests passed!\n";
+    std::cout << "\nAll " << 22 << " functionality tests passed!\n";
 
     PrintPerformanceHeader();