diff --git a/src/CryptoNoteCore/Blockchain.cpp b/src/CryptoNoteCore/Blockchain.cpp
index 59774826..8f236a7a 100644
--- a/src/CryptoNoteCore/Blockchain.cpp
+++ b/src/CryptoNoteCore/Blockchain.cpp
@@ -1,12 +1,16 @@
 // Copyright (c) 2011-2017 The Cryptonote developers
+// Copyright (c) 2016-2018, The Karbo developers
 // Copyright (c) 2018 The Circle Foundation
+
 // Distributed under the MIT/X11 software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include "Blockchain.h"
 
 #include <algorithm>
+#include <numeric>
 #include <cstdio>
+#include <cmath>
 #include <boost/foreach.hpp>
 #include "Common/Math.h"
 #include "Common/int-util.h"
@@ -16,6 +20,7 @@
 #include "Rpc/CoreRpcServerCommandsDefinitions.h"
 #include "Serialization/BinarySerializationTools.h"
 #include "CryptoNoteTools.h"
+#include "TransactionExtra.h"
 
 using namespace Logging;
 using namespace Common;
@@ -1183,6 +1188,13 @@ bool Blockchain::handle_alternative_block(const Block& b, const Crypto::Hash& id
       return false;
     }
 
+    // Disable merged mining
+    TransactionExtraMergeMiningTag mmTag;
+    if (getMergeMiningTagFromExtra(bei.bl.baseTransaction.extra, mmTag) && bei.height >= CryptoNote::parameters::UPGRADE_HEIGHT_V6) {
+      logger(ERROR, BRIGHT_RED) << "Merge mining tag was found in extra of miner transaction";
+      return false;
+    }
+
     // Always check PoW for alternative blocks
     m_is_in_checkpoint_zone = false;
     difficulty_type current_diff = get_next_difficulty_for_alternative_chain(alt_chain, bei);
@@ -1918,6 +1930,17 @@ bool Blockchain::pushBlock(const Block& blockData, const std::vector<Transaction
     return false;
   }
 
+  // Disable merged mining
+  uint32_t height = 0;
+  TransactionExtraMergeMiningTag mmTag;
+  if (m_blockIndex.getBlockHeight(blockHash, height)) {
+    if (getMergeMiningTagFromExtra(blockData.baseTransaction.extra, mmTag) && height >= CryptoNote::parameters::UPGRADE_HEIGHT_V6) {
+      logger(ERROR, BRIGHT_RED) << "Merge mining tag was found in extra of miner transaction";
+      return false;
+    }
+  }
+
+
   if (blockData.previousBlockHash != getTailId()) {
     logger(INFO, BRIGHT_WHITE) <<
       "Block " << blockHash << " has wrong previousBlockHash: " << blockData.previousBlockHash << ", expected: " << getTailId();
diff --git a/src/CryptoNoteCore/Checkpoints.cpp b/src/CryptoNoteCore/Checkpoints.cpp
index 9c88a1f3..6d595d6a 100644
--- a/src/CryptoNoteCore/Checkpoints.cpp
+++ b/src/CryptoNoteCore/Checkpoints.cpp
@@ -1,5 +1,7 @@
 // Copyright (c) 2011-2017 The Cryptonote developers
 // Copyright (c) 2018 The Circle Foundation
+// Copyright (c) 2016-2018, The Karbo developers
+
 // Distributed under the MIT/X11 software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
diff --git a/src/CryptoNoteCore/TransactionExtra.h b/src/CryptoNoteCore/TransactionExtra.h
index 89233d4b..58c8d331 100644
--- a/src/CryptoNoteCore/TransactionExtra.h
+++ b/src/CryptoNoteCore/TransactionExtra.h
@@ -1,5 +1,6 @@
 // Copyright (c) 2011-2017 The Cryptonote developers
 // Copyright (c) 2018 The Circle Foundation
+
 // Distributed under the MIT/X11 software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -89,6 +90,7 @@ bool appendMergeMiningTagToExtra(std::vector<uint8_t>& tx_extra, const Transacti
 bool append_message_to_extra(std::vector<uint8_t>& tx_extra, const tx_extra_message& message);
 std::vector<std::string> get_messages_from_extra(const std::vector<uint8_t>& extra, const Crypto::PublicKey &txkey, const Crypto::SecretKey *recepient_secret_key);
 void appendTTLToExtra(std::vector<uint8_t>& tx_extra, uint64_t ttl);
+bool getMergeMiningTagFromExtra(const std::vector<uint8_t>& tx_extra, TransactionExtraMergeMiningTag& mm_tag);
 
 bool createTxExtraWithPaymentId(const std::string& paymentIdString, std::vector<uint8_t>& extra);
 //returns false if payment id is not found or parse error