diff --git a/src/packet/packet.js b/src/packet/packet.js
index c40082f5..6eea1209 100644
--- a/src/packet/packet.js
+++ b/src/packet/packet.js
@@ -308,6 +308,19 @@ export class UnsupportedError extends Error {
   }
 }
 
+// unknown packet types are handled differently depending on the packet criticality
+export class UnknownPacketError extends UnsupportedError {
+  constructor(...params) {
+    super(...params);
+
+    if (Error.captureStackTrace) {
+      Error.captureStackTrace(this, UnsupportedError);
+    }
+
+    this.name = 'UnknownPacketError';
+  }
+}
+
 export class UnparseablePacket {
   constructor(tag, rawContent) {
     this.tag = tag;
diff --git a/src/packet/packetlist.js b/src/packet/packetlist.js
index 6207ef99..2c8e2a08 100644
--- a/src/packet/packetlist.js
+++ b/src/packet/packetlist.js
@@ -4,7 +4,8 @@ import {
   writeTag, writeHeader,
   writePartialLength, writeSimpleLength,
   UnparseablePacket,
-  UnsupportedError
+  UnsupportedError,
+  UnknownPacketError
 } from './packet';
 import util from '../util';
 import enums from '../enums';
@@ -25,7 +26,7 @@ export function newPacketFromTag(tag, allowedPackets) {
     try {
       packetType = enums.read(enums.packet, tag);
     } catch (e) {
-      throw new UnsupportedError(`Unknown packet type with tag: ${tag}`);
+      throw new UnknownPacketError(`Unknown packet type with tag: ${tag}`);
     }
     throw new Error(`Packet not allowed in this context: ${packetType}`);
   }
@@ -87,6 +88,17 @@ class PacketList extends Array {
               await packet.read(parsed.packet, config);
               await writer.write(packet);
             } catch (e) {
+              // If an implementation encounters a critical packet where the packet type is unknown in a packet sequence,
+              // it MUST reject the whole packet sequence. On the other hand, an unknown non-critical packet MUST be ignored.
+              // Packet Tags from 0 to 39 are critical. Packet Tags from 40 to 63 are non-critical.
+              if (e instanceof UnknownPacketError) {
+                if (parsed.tag <= 39) {
+                  await writer.abort(e);
+                } else {
+                  return;
+                }
+              }
+
               const throwUnsupportedError = !config.ignoreUnsupportedPackets && e instanceof UnsupportedError;
               const throwMalformedError = !config.ignoreMalformedPackets && !(e instanceof UnsupportedError);
               if (throwUnsupportedError || throwMalformedError || supportsStreaming(parsed.tag)) {
diff --git a/test/general/packet.js b/test/general/packet.js
index 0e1cdaab..696d2846 100644
--- a/test/general/packet.js
+++ b/test/general/packet.js
@@ -1514,6 +1514,20 @@ kePFjAnu9cpynKXu3usf8+FuBw2zLsg1Id1n7ttxoAte416KjBN9lFBt8mcu
       ).to.be.rejectedWith(/Unsupported S2K type/);
     });
 
+    it('Throws on critical packet even with tolerant mode enabled', async function() {
+      const unknownPacketTag39 = util.hexToUint8Array('e70a750064bf943d6e756c6c'); // critical tag
+
+      await expect(openpgp.PacketList.fromBinary(unknownPacketTag39, allAllowedPackets, { ...openpgp.config, ignoreUnsupportedPackets: false, ignoreMalformedPackets: false })).to.be.rejectedWith(/Unknown packet type/);
+      await expect(openpgp.PacketList.fromBinary(unknownPacketTag39, allAllowedPackets, { ...openpgp.config, ignoreUnsupportedPackets: true, ignoreMalformedPackets: true })).to.be.rejectedWith(/Unknown packet type/);
+    });
+
+    it('Ignores non-critical packet even with tolerant mode disabled', async function() {
+      const unknownPacketTag63 = util.hexToUint8Array('ff0a750064bf943d6e756c6c'); // non-critical tag
+
+      await expect(openpgp.PacketList.fromBinary(unknownPacketTag63, allAllowedPackets, { ...openpgp.config, ignoreUnsupportedPackets: false, ignoreMalformedPackets: false })).to.eventually.have.length(0);
+      await expect(openpgp.PacketList.fromBinary(unknownPacketTag63, allAllowedPackets, { ...openpgp.config, ignoreUnsupportedPackets: true, ignoreMalformedPackets: true })).to.eventually.have.length(0);
+    });
+
     it('Throws on disallowed packet even with tolerant mode enabled', async function() {
       const packets = new openpgp.PacketList();
       packets.push(new openpgp.LiteralDataPacket());