diff --git a/impit/src/errors.rs b/impit/src/errors.rs
index d815590b..97d01570 100644
--- a/impit/src/errors.rs
+++ b/impit/src/errors.rs
@@ -84,7 +84,7 @@ pub enum ImpitError {
     UrlMissingHostnameError(String),
     #[error("The URL uses an unsupported protocol (`{0}`). Currently, only HTTP and HTTPS are supported.")]
     UrlProtocolError(String),
-    #[error("The request was made with http3_prior_knowledge, but HTTP/3 usage wasn't enabled.")]
+    #[error("The request was made with http3_prior_knowledge, but HTTP/3 usage wasn't enabled (or was disabled because a proxy is set).")]
     Http3Disabled,
     #[error("The request method `{0}` is invalid. Only GET, POST, PUT, DELETE, PATCH, HEAD, OPTIONS and TRACE are supported.")]
     InvalidMethod(String),
diff --git a/impit/src/impit.rs b/impit/src/impit.rs
index 559967b0..9a8164cc 100644
--- a/impit/src/impit.rs
+++ b/impit/src/impit.rs
@@ -1,7 +1,7 @@
 #[cfg(feature = "http3")]
 use tokio::sync::RwLock;
 
-use log::debug;
+use log::{debug, warn};
 use reqwest::{cookie::CookieStore, header::HeaderMap, Method, Response, Version};
 use std::{fmt::Debug, net::IpAddr, sync::Arc, time::Duration};
 use url::Url;
@@ -179,6 +179,10 @@ impl<CookieStoreImpl: CookieStore + 'static> ImpitBuilder<CookieStoreImpl> {
     ///
     /// Requires the `http3` feature of this crate (and the `--cfg reqwest_unstable` rustflag).
     ///
+    /// HTTP/3 is never used with a proxy, set either with [`with_proxy`](Self::with_proxy) or in the
+    /// `HTTPS_PROXY` or `ALL_PROXY` environment variables: proxies only carry TCP, so QUIC requests would
+    /// skip the proxy and go out from this machine's IP address. With a proxy, the client uses HTTP/2.
+    ///
     /// Note that this feature is experimental and may not work as expected with all servers.
     #[cfg(feature = "http3")]
     pub fn with_http3(mut self) -> Self {
@@ -231,6 +235,15 @@ impl<CookieStoreImpl: CookieStore + 'static> ImpitBuilder<CookieStoreImpl> {
         self
     }
 
+    /// Whether requests go through a proxy: the one set with [`with_proxy`](Self::with_proxy), or one
+    /// that `reqwest` reads from the environment for HTTPS URLs.
+    fn uses_proxy(&self) -> bool {
+        !self.proxy_url.is_empty()
+            || ["HTTPS_PROXY", "https_proxy", "ALL_PROXY", "all_proxy"]
+                .iter()
+                .any(|name| std::env::var_os(name).is_some_and(|value| !value.is_empty()))
+    }
+
     /// Builds the [`Impit`] instance.
     pub fn build(self) -> Result<Impit<CookieStoreImpl>, ImpitError> {
         Impit::new(self)
@@ -322,7 +335,12 @@ impl<CookieStoreImpl: CookieStore + 'static> Impit<CookieStoreImpl> {
     }
 
     /// Creates a new [`Impit`] instance based on the options stored in the [`ImpitBuilder`] instance.
-    fn new(config: ImpitBuilder<CookieStoreImpl>) -> Result<Self, ImpitError> {
+    fn new(mut config: ImpitBuilder<CookieStoreImpl>) -> Result<Self, ImpitError> {
+        if config.max_http_version == Version::HTTP_3 && config.uses_proxy() {
+            warn!("HTTP/3 is disabled because a proxy is set: QUIC requests would skip the proxy");
+            config.max_http_version = Version::HTTP_2;
+        }
+
         let mut h3_client: Option<reqwest::Client> = None;
         let mut base_client = Self::new_reqwest_client(&config)?;
 
@@ -742,4 +760,19 @@ mod tests {
             Err(ImpitError::Http3Disabled)
         ));
     }
+
+    #[cfg(feature = "http3")]
+    #[tokio::test]
+    async fn http3_is_disabled_with_a_proxy() {
+        let impit = Impit::<Jar>::builder()
+            .with_http3()
+            .with_proxy("http://127.0.0.1:9".to_string())
+            .build()
+            .unwrap();
+        assert!(impit.h3_client.is_none());
+        assert!(matches!(
+            get_with_http3_prior_knowledge(&impit).await,
+            Err(ImpitError::Http3Disabled)
+        ));
+    }
 }