diff --git a/cmd/geth/config.go b/cmd/geth/config.go
index 0a63a027760d..e2c107ffc541 100644
--- a/cmd/geth/config.go
+++ b/cmd/geth/config.go
@@ -20,6 +20,7 @@ import (
 	"bufio"
 	"errors"
 	"fmt"
+	"math/big"
 	"os"
 	"reflect"
 	"unicode"
@@ -150,6 +151,9 @@ func enableWhisper(ctx *cli.Context) bool {
 
 func makeFullNode(ctx *cli.Context) *node.Node {
 	stack, cfg := makeConfigNode(ctx)
+	if ctx.GlobalIsSet(utils.OverrideIstanbulFlag.Name) {
+		cfg.Eth.OverrideIstanbul = new(big.Int).SetUint64(ctx.GlobalUint64(utils.OverrideIstanbulFlag.Name))
+	}
 	utils.RegisterEthService(stack, &cfg.Eth)
 
 	if ctx.GlobalBool(utils.DashboardEnabledFlag.Name) {
diff --git a/cmd/geth/main.go b/cmd/geth/main.go
index 4c51f092d5e4..019eee605f87 100644
--- a/cmd/geth/main.go
+++ b/cmd/geth/main.go
@@ -69,6 +69,7 @@ var (
 		utils.ExternalSignerFlag,
 		utils.NoUSBFlag,
 		utils.SmartCardDaemonPathFlag,
+		utils.OverrideIstanbulFlag,
 		utils.DashboardEnabledFlag,
 		utils.DashboardAddrFlag,
 		utils.DashboardPortFlag,
diff --git a/cmd/utils/flags.go b/cmd/utils/flags.go
index 7e46043ff887..d553b662c8d8 100644
--- a/cmd/utils/flags.go
+++ b/cmd/utils/flags.go
@@ -223,6 +223,10 @@ var (
 		Name:  "whitelist",
 		Usage: "Comma separated block number-to-hash mappings to enforce (<number>=<hash>)",
 	}
+	OverrideIstanbulFlag = cli.Uint64Flag{
+		Name:  "override.istanbul",
+		Usage: "Manually specify Istanbul fork-block, overriding the bundled setting",
+	}
 	// Light server and client settings
 	LightLegacyServFlag = cli.IntFlag{ // Deprecated in favor of light.serve, remove in 2021
 		Name:  "lightserv",
diff --git a/core/genesis.go b/core/genesis.go
index 5d6311b32471..8261c18cc9f0 100644
--- a/core/genesis.go
+++ b/core/genesis.go
@@ -152,6 +152,10 @@ func (e *GenesisMismatchError) Error() string {
 //
 // The returned chain configuration is never nil.
 func SetupGenesisBlock(db ethdb.Database, genesis *Genesis) (*params.ChainConfig, common.Hash, error) {
+	return SetupGenesisBlockWithOverride(db, genesis, nil)
+}
+
+func SetupGenesisBlockWithOverride(db ethdb.Database, genesis *Genesis, overrideIstanbul *big.Int) (*params.ChainConfig, common.Hash, error) {
 	if genesis != nil && genesis.Config == nil {
 		return params.AllEthashProtocolChanges, common.Hash{}, errGenesisNoConfig
 	}
@@ -200,6 +204,9 @@ func SetupGenesisBlock(db ethdb.Database, genesis *Genesis) (*params.ChainConfig
 
 	// Get the existing chain configuration.
 	newcfg := genesis.configOrDefault(stored)
+	if overrideIstanbul != nil {
+		newcfg.IstanbulBlock = overrideIstanbul
+	}
 	storedcfg := rawdb.ReadChainConfig(db, stored)
 	if storedcfg == nil {
 		log.Warn("Found genesis block without chain config")
diff --git a/eth/backend.go b/eth/backend.go
index dc4ff8ade8db..ce37541f4d1f 100644
--- a/eth/backend.go
+++ b/eth/backend.go
@@ -137,7 +137,7 @@ func New(ctx *node.ServiceContext, config *Config) (*Ethereum, error) {
 	if err != nil {
 		return nil, err
 	}
-	chainConfig, genesisHash, genesisErr := core.SetupGenesisBlock(chainDb, config.Genesis)
+	chainConfig, genesisHash, genesisErr := core.SetupGenesisBlockWithOverride(chainDb, config.Genesis, config.OverrideIstanbul)
 	if _, ok := genesisErr.(*params.ConfigCompatError); genesisErr != nil && !ok {
 		return nil, genesisErr
 	}
diff --git a/eth/config.go b/eth/config.go
index 688787227600..5094a533bf4b 100644
--- a/eth/config.go
+++ b/eth/config.go
@@ -154,4 +154,7 @@ type Config struct {
 
 	// CheckpointOracle is the configuration for checkpoint oracle.
 	CheckpointOracle *params.CheckpointOracleConfig `toml:",omitempty"`
+
+	// Istanbul block override (TODO: remove after the fork)
+	OverrideIstanbul *big.Int
 }
diff --git a/les/client.go b/les/client.go
index 461baf645ef0..b367681f3768 100644
--- a/les/client.go
+++ b/les/client.go
@@ -72,7 +72,7 @@ func New(ctx *node.ServiceContext, config *eth.Config) (*LightEthereum, error) {
 	if err != nil {
 		return nil, err
 	}
-	chainConfig, genesisHash, genesisErr := core.SetupGenesisBlock(chainDb, config.Genesis)
+	chainConfig, genesisHash, genesisErr := core.SetupGenesisBlockWithOverride(chainDb, config.Genesis, config.OverrideIstanbul)
 	if _, isCompat := genesisErr.(*params.ConfigCompatError); genesisErr != nil && !isCompat {
 		return nil, genesisErr
 	}