diff --git a/src/main.cpp b/src/main.cpp
index cb41afc96..2a6fa7d0b 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -1853,11 +1853,18 @@ bool ConnectBlock(const CBlock& block, CValidationState& state, CBlockIndex* pin
 
     unsigned int flags = fStrictPayToScriptHash ? SCRIPT_VERIFY_P2SH : SCRIPT_VERIFY_NONE;
 
-    // Start enforcing the DERSIG (BIP66) rules, for block.nVersion=3 blocks, when 75% of the network has upgraded:
+    // Start enforcing the DERSIG (BIP66) rules, for block.nVersion=3 blocks,
+    // when 75% of the network has upgraded:
     if (block.nVersion >= 3 && IsSuperMajority(3, pindex->pprev, chainparams.GetConsensus().nMajorityEnforceBlockUpgrade, chainparams.GetConsensus())) {
         flags |= SCRIPT_VERIFY_DERSIG;
     }
 
+    // Start enforcing CHECKLOCKTIMEVERIFY, (BIP65) for block.nVersion=4
+    // blocks, when 75% of the network has upgraded:
+    if (block.nVersion >= 4 && IsSuperMajority(4, pindex->pprev, chainparams.GetConsensus().nMajorityEnforceBlockUpgrade, chainparams.GetConsensus())) {
+        flags |= SCRIPT_VERIFY_CHECKLOCKTIMEVERIFY;
+    }
+
     CBlockUndo blockundo;
 
     CCheckQueueControl<CScriptCheck> control(fScriptChecks && nScriptCheckThreads ? &scriptcheckqueue : NULL);
@@ -2788,6 +2795,11 @@ bool ContextualCheckBlockHeader(const CBlockHeader& block, CValidationState& sta
         return state.Invalid(error("%s : rejected nVersion=2 block", __func__),
                              REJECT_OBSOLETE, "bad-version");
 
+    // Reject block.nVersion=3 blocks when 95% (75% on testnet) of the network has upgraded:
+    if (block.nVersion < 4 && IsSuperMajority(4, pindexPrev, consensusParams.nMajorityRejectBlockOutdated, consensusParams))
+        return state.Invalid(error("%s : rejected nVersion=3 block", __func__),
+                             REJECT_OBSOLETE, "bad-version");
+
     return true;
 }
 
diff --git a/src/primitives/block.h b/src/primitives/block.h
index 59f46deb1..90b8904ab 100644
--- a/src/primitives/block.h
+++ b/src/primitives/block.h
@@ -21,7 +21,7 @@ class CBlockHeader
 {
 public:
     // header
-    static const int32_t CURRENT_VERSION=3;
+    static const int32_t CURRENT_VERSION=4;
     int32_t nVersion;
     uint256 hashPrevBlock;
     uint256 hashMerkleRoot;
diff --git a/src/script/bitcoinconsensus.h b/src/script/bitcoinconsensus.h
index 032057779..a48ff1e18 100644
--- a/src/script/bitcoinconsensus.h
+++ b/src/script/bitcoinconsensus.h
@@ -44,9 +44,10 @@ typedef enum bitcoinconsensus_error_t
 /** Script verification flags */
 enum
 {
-    bitcoinconsensus_SCRIPT_FLAGS_VERIFY_NONE      = 0,
-    bitcoinconsensus_SCRIPT_FLAGS_VERIFY_P2SH      = (1U << 0), // evaluate P2SH (BIP16) subscripts
-    bitcoinconsensus_SCRIPT_FLAGS_VERIFY_DERSIG    = (1U << 2), // enforce strict DER (BIP66) compliance
+    bitcoinconsensus_SCRIPT_FLAGS_VERIFY_NONE                = 0,
+    bitcoinconsensus_SCRIPT_FLAGS_VERIFY_P2SH                = (1U << 0), // evaluate P2SH (BIP16) subscripts
+    bitcoinconsensus_SCRIPT_FLAGS_VERIFY_DERSIG              = (1U << 2), // enforce strict DER (BIP66) compliance
+    bitcoinconsensus_SCRIPT_FLAGS_VERIFY_CHECKLOCKTIMEVERIFY = (1U << 9), // enable CHECKLOCKTIMEVERIFY (BIP65)
 };
 
 /// Returns 1 if the input nIn of the serialized transaction pointed to by