diff --git a/beacon_node/execution_layer/src/test_utils/execution_block_generator.rs b/beacon_node/execution_layer/src/test_utils/execution_block_generator.rs
index 72a883e1ee..d474374509 100644
--- a/beacon_node/execution_layer/src/test_utils/execution_block_generator.rs
+++ b/beacon_node/execution_layer/src/test_utils/execution_block_generator.rs
@@ -50,7 +50,7 @@ impl ExecutionBlockGenerator {
         }
     }
 
-    pub fn insert_merge_block(&mut self, number: u64, block_hash: Hash256) -> Result<(), String> {
+    pub fn insert_pos_block(&mut self, number: u64) -> Result<(), String> {
         if number <= self.terminal_block_number {
             return Err(format!(
                 "cannot insert block {} as it is prior to terminal block {}",
@@ -58,6 +58,13 @@ impl ExecutionBlockGenerator {
             ));
         }
 
+        let time_based_block = self.block_number_at(self.seconds_since_genesis);
+        if time_based_block < self.terminal_block_number {
+            if number > time_based_block {
+                return Err(format!("it is too early to insert block {}", number));
+            }
+        }
+
         let next_block = self
             .latest_merge_block
             .unwrap_or(self.terminal_block_number)
@@ -148,7 +155,7 @@ mod test {
     use super::*;
 
     #[test]
-    fn simple() {
+    fn pow_chain_only() {
         const TERMINAL_DIFFICULTY: u64 = 10;
         const TERMINAL_BLOCK: u64 = 10;
         const DIFFICULTY_INCREMENT: u64 = 1;
@@ -201,4 +208,37 @@ mod test {
             generator.increment_seconds_since_genesis(1);
         }
     }
+
+    #[test]
+    fn pos_blocks() {
+        const TERMINAL_DIFFICULTY: u64 = 10;
+        const TERMINAL_BLOCK: u64 = 10;
+
+        let mut generator = ExecutionBlockGenerator::new(TERMINAL_DIFFICULTY, TERMINAL_BLOCK);
+
+        let penultimate_pow_block = generator.terminal_block_number.checked_sub(1).unwrap();
+        let last_pow_block = generator.terminal_block_number;
+        let first_pos_block = generator.terminal_block_number + 1;
+        let second_pos_block = first_pos_block + 1;
+
+        generator.set_clock_for_block_number(penultimate_pow_block);
+
+        assert!(generator.block_by_number(last_pow_block).is_none());
+
+        assert!(generator.insert_pos_block(first_pos_block).is_err());
+
+        generator.set_clock_for_block_number(last_pow_block);
+
+        generator.block_by_number(last_pow_block).unwrap();
+
+        assert!(generator.block_by_number(first_pos_block).is_none());
+
+        generator.insert_pos_block(first_pos_block).unwrap();
+
+        generator.block_by_number(first_pos_block).unwrap();
+
+        assert!(generator.insert_pos_block(first_pos_block).is_err());
+
+        generator.insert_pos_block(second_pos_block).unwrap();
+    }
 }