diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5b12ca4a..93dc1e6c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,9 @@
+Version 340:
+
+*  Expect header field with the "100-continue" is handled in upgrade.
+
+--------------------------------------------------------------------------------
+
 Version 339:
 
 * BOOST_BEAST_USE_STD_STRING_VIEW is replaced by boost/core string_view.
diff --git a/example/doc/http_examples.hpp b/example/doc/http_examples.hpp
index d17697a5..dac3e248 100644
--- a/example/doc/http_examples.hpp
+++ b/example/doc/http_examples.hpp
@@ -137,7 +137,7 @@ receive_expect_100_continue(
         return;
 
     // Check for the Expect field value
-    if(parser.get()[field::expect] == "100-continue")
+    if(beast::iequals(parser.get()[field::expect], "100-continue"))
     {
         // send 100 response
         response<empty_body> res;
diff --git a/include/boost/beast/websocket/impl/accept.hpp b/include/boost/beast/websocket/impl/accept.hpp
index 3eef063a..4f8dbcc7 100644
--- a/include/boost/beast/websocket/impl/accept.hpp
+++ b/include/boost/beast/websocket/impl/accept.hpp
@@ -171,6 +171,8 @@ class stream<NextLayer, deflateSupported>::response_op
     boost::weak_ptr<impl_type> wp_;
     error_code result_; // must come before res_
     response_type& res_;
+    http::response<http::empty_body> res_100_;
+    bool needs_res_100_{false};
 
 public:
     template<
@@ -192,6 +194,15 @@ public:
         , res_(beast::allocate_stable<response_type>(*this,
             sp->build_response(req, decorator, result_)))
     {
+        auto itr = req.find(http::field::expect);
+        if (itr != req.end() && iequals(itr->value(), "100-continue")) // do
+        {
+            res_100_.version(res_.version());
+            res_100_.set(http::field::server, res_[http::field::server]);
+            res_100_.result(http::status::continue_);
+            res_100_.prepare_payload();
+            needs_res_100_ = true;
+        }
         (*this)({}, 0, cont);
     }
 
@@ -213,6 +224,16 @@ public:
             impl.change_status(status::handshake);
             impl.update_timer(this->get_executor());
 
+            if (needs_res_100_)
+            {
+                BOOST_ASIO_CORO_YIELD
+                {
+                    BOOST_ASIO_HANDLER_LOCATION((__FILE__, __LINE__, "websocket::async_accept"));
+                    http::async_write(
+                            impl.stream(), res_100_, std::move(*this));
+                }
+            }
+
             // Send response
             BOOST_ASIO_CORO_YIELD
             {
@@ -323,6 +344,7 @@ public:
                 // the handler.
                 auto const req = p_.release();
                 auto const decorator = d_;
+
                 response_op<Handler>(
                     this->release_handler(),
                         sp, req, decorator, true);
@@ -417,6 +439,20 @@ do_accept(
 
     error_code result;
     auto const res = impl_->build_response(req, decorator, result);
+
+    auto itr = req.find(http::field::expect);
+    if (itr != req.end() && iequals(itr->value(), "100-continue")) // do
+    {
+        http::response<http::empty_body> res_100;
+        res_100.version(res.version());
+        res_100.set(http::field::server, res[http::field::server]);
+        res_100.result(http::status::continue_);
+        res_100.prepare_payload();
+        http::write(impl_->stream(), res_100, ec);
+        if (ec)
+            return;
+    }
+
     http::write(impl_->stream(), res, ec);
     if(ec)
         return;
diff --git a/test/beast/websocket/accept.cpp b/test/beast/websocket/accept.cpp
index ab5a0f70..ca6d4357 100644
--- a/test/beast/websocket/accept.cpp
+++ b/test/beast/websocket/accept.cpp
@@ -816,6 +816,87 @@ public:
         }
     }
 
+    void testIssue264Sync()
+    {
+        net::io_context ioc;
+        using tcp = net::ip::tcp;
+
+        stream<tcp::socket> ws1(ioc);
+        tcp::socket s2(ioc);
+        test::connect(ws1.next_layer(), s2);
+
+        http::request<http::empty_body> req{http::verb::get, "/api", 11};
+        req.set(http::field::connection, "upgrade");
+        req.set(http::field::upgrade, "websocket");
+        req.set(http::field::expect, "100-continue");
+        req.set(http::field::host, "test");
+        req.set(http::field::sec_websocket_version, "13");
+        req.set(http::field::sec_websocket_key, "1234");
+
+        req.prepare_payload();
+
+        std::thread thr{
+            [&]
+            {
+                beast::error_code ec;
+                ws1.accept(ec);
+                BEAST_EXPECTS(!ec, ec.message());
+            }};
+
+        http::async_write(s2, req, test::success_handler());
+        http::response<http::empty_body> res1, res2;
+
+        flat_buffer buf;
+        http::async_read(s2, buf, res1,
+             [&](error_code ec, std::size_t)
+             {
+                 BEAST_EXPECTS(!ec, ec.message());
+                 BEAST_EXPECTS(res1.result() == http::status::continue_, obsolete_reason(res1.result()));
+                 if (res1.result() == http::status::continue_)
+                     http::async_read(s2, buf, res2, test::success_handler());
+            });
+
+        test::run_for(ioc, std::chrono::seconds(1));
+        thr.join();
+    }
+
+
+    void testIssue264Async()
+    {
+        net::io_context ioc;
+        using tcp = net::ip::tcp;
+
+        stream<tcp::socket> ws1(ioc);
+        tcp::socket s2(ioc);
+        test::connect(ws1.next_layer(), s2);
+
+        http::request<http::empty_body> req{http::verb::get, "/api", 11};
+        req.set(http::field::connection, "upgrade");
+        req.set(http::field::upgrade, "websocket");
+        req.set(http::field::expect, "100-continue");
+        req.set(http::field::host, "test");
+        req.set(http::field::sec_websocket_version, "13");
+        req.set(http::field::sec_websocket_key, "1234");
+
+        req.prepare_payload();
+
+        http::async_write(s2, req, test::success_handler());
+        http::response<http::empty_body> res1, res2;
+
+        ws1.async_accept(test::success_handler());
+        flat_buffer buf;
+        http::async_read(s2, buf, res1,
+                         [&](error_code ec, std::size_t)
+                         {
+                             BEAST_EXPECTS(!ec, ec.message());
+                             BEAST_EXPECTS(res1.result() == http::status::continue_, obsolete_reason(res1.result()));
+                             if (res1.result() == http::status::continue_)
+                                 http::async_read(s2, buf, res2, test::success_handler());
+                         });
+
+        test::run_for(ioc, std::chrono::seconds(1));
+    }
+
 #if BOOST_ASIO_HAS_CO_AWAIT
     void testAwaitableCompiles(
         stream<net::ip::tcp::socket>& s,
@@ -850,6 +931,8 @@ public:
 #if BOOST_ASIO_HAS_CO_AWAIT
         boost::ignore_unused(&accept_test::testAwaitableCompiles);
 #endif
+        testIssue264Sync();
+        testIssue264Async();
     }
 };