diff --git a/frontend/web/components/EnvironmentReadyChecker.tsx b/frontend/web/components/EnvironmentReadyChecker.tsx
index 9e32893f2bf9..6f065fe66be1 100644
--- a/frontend/web/components/EnvironmentReadyChecker.tsx
+++ b/frontend/web/components/EnvironmentReadyChecker.tsx
@@ -1,8 +1,9 @@
-import { PropsWithChildren, useEffect, useState } from 'react'
+import { PropsWithChildren } from 'react'
 import { useGetEnvironmentQuery } from 'common/services/useEnvironment'
 import { useRouteMatch } from 'react-router-dom'
 
 interface RouteParams {
+  projectId?: string
   environmentId?: string
 }
 
@@ -10,44 +11,61 @@ type EnvironmentReadyCheckerType = {
   children: React.ReactNode
 }
 
+const POLL_INTERVAL_MS = 1000
+
+const LoadingState = () => (
+  <div className='text-center'>
+    <Loader />
+  </div>
+)
+
+const CreatingState = () => (
+  <div className='container'>
+    <div className='d-flex flex-column h-100 flex-1 justify-content-center align-items-center'>
+      <Loader />
+      <h3>Preparing your environment</h3>
+      <p>We are setting up your new environment...</p>
+    </div>
+  </div>
+)
+
+const NotFoundState = () => (
+  <div className='app-container container'>
+    <h3 className='pt-5'>Environment not found</h3>
+    <p>
+      This environment may have been deleted, or you may not have permission to
+      access it. Check the URL and try again.
+    </p>
+  </div>
+)
+
 const EnvironmentReadyChecker = ({
   children,
 }: PropsWithChildren<EnvironmentReadyCheckerType>) => {
   const match = useRouteMatch<RouteParams>()
-  const [environmentCreated, setEnvironmentCreated] = useState(false)
+  const environmentId = match?.params?.environmentId
+  const projectId = match?.params?.projectId
+  // 'create' is the new-env form route sentinel, not an env ID.
+  const hasEnvironmentId = !!environmentId && environmentId !== 'create'
 
-  const { data, isLoading } = useGetEnvironmentQuery(
-    {
-      id: match?.params?.environmentId || '',
-    },
+  const { data, isError } = useGetEnvironmentQuery(
+    { id: environmentId || '' },
     {
-      pollingInterval: 1000,
-      skip: !match?.params?.environmentId || environmentCreated,
+      pollingInterval: data?.is_creating ? POLL_INTERVAL_MS : 0,
+      skip: !hasEnvironmentId,
     },
   )
-  useEffect(() => {
-    if (!!data && !data?.is_creating) {
-      setEnvironmentCreated(true)
-    }
-  }, [data])
-  if (!match?.params?.environmentId) {
-    return children
-  }
-  return isLoading ? (
-    <div className='text-center'>
-      <Loader />
-    </div>
-  ) : data?.is_creating ? (
-    <div className='container'>
-      <div className='d-flex flex-column h-100 flex-1 justify-content-center align-items-center'>
-        <Loader />
-        <h3>Preparing your environment</h3>
-        <p>We are setting up your new environment...</p>
-      </div>
-    </div>
-  ) : (
-    children
-  )
+
+  // Env-by-api_key endpoint isn't project-scoped — verify the match client-side.
+  const wrongProject =
+    !!data && !!projectId && data.project !== Number(projectId)
+  const environmentNotFound = hasEnvironmentId && (wrongProject || isError)
+
+  if (!hasEnvironmentId) return children
+  if (environmentNotFound) return <NotFoundState />
+  if (!data) return <LoadingState />
+  if (data.is_creating) return <CreatingState />
+  return children
 }
 
 export default EnvironmentReadyChecker
diff --git a/frontend/web/components/navigation/EnvironmentAside.tsx b/frontend/web/components/navigation/EnvironmentAside.tsx
index 07fa75f28111..45ec3eda32a1 100644
--- a/frontend/web/components/navigation/EnvironmentAside.tsx
+++ b/frontend/web/components/navigation/EnvironmentAside.tsx
@@ -16,11 +16,12 @@ import AppActions from 'common/dispatcher/app-actions'
 import EnvironmentSelect from 'components/EnvironmentSelect'
 import { components } from 'react-select'
 import BuildVersion from 'components/BuildVersion'
+import { useGetEnvironmentsQuery } from 'common/services/useEnvironment'
 import { useGetHealthEventsQuery } from 'common/services/useHealthEvents'
 import Constants from 'common/constants'
 import EnvironmentNavbar from './navbars/EnvironmentNavbar'
 import OverflowNav from './OverflowNav'
-import { ProjectPermission } from 'common/types/permissions.types';
+import { ProjectPermission } from 'common/types/permissions.types'
 
 type HomeAsideType = {
   environmentId: string
@@ -102,6 +103,8 @@ const CustomSingleValue = ({ hasWarning, ...rest }: CustomSingleValueProps) => {
 
 const EnvironmentAside: FC<HomeAsideType> = ({ environmentId, projectId }) => {
   const history = useHistory()
+  const { data: environments, isSuccess: environmentsLoaded } =
+    useGetEnvironmentsQuery({ projectId }, { skip: !projectId })
   const { data: healthEvents } = useGetHealthEventsQuery(
     { projectId: projectId },
     { skip: !projectId },
@@ -126,9 +129,18 @@ const EnvironmentAside: FC<HomeAsideType> = ({ environmentId, projectId }) => {
       ? null
       : (ProjectStore.getEnvironment(environmentId) as any)
 
+  const environmentNotFound =
+    environmentId !== 'create' &&
+    environmentsLoaded &&
+    !environments?.results?.some((env) => env.api_key === environmentId)
+
   const onProjectSave = () => {
     AppActions.refreshOrganisation()
   }
+
+  if (environmentNotFound) {
+    return null
+  }
   return (
     <>
       <OrganisationProvider>
diff --git a/frontend/web/components/navigation/navbars/EnvironmentNavbar.tsx b/frontend/web/components/navigation/navbars/EnvironmentNavbar.tsx
index 3dccf9503c7d..15ee18c8d47d 100644
--- a/frontend/web/components/navigation/navbars/EnvironmentNavbar.tsx
+++ b/frontend/web/components/navigation/navbars/EnvironmentNavbar.tsx
@@ -21,30 +21,36 @@ const EnvironmentNavbar: FC<EnvironmentNavType> = ({
 }) => {
   const date = useRef(moment().toISOString())
 
-  const { data: environments } = useGetEnvironmentsQuery(
+  const { data: environments, isSuccess: environmentsLoaded } =
+    useGetEnvironmentsQuery({ projectId }, { skip: !projectId })
+
+  const environment = environments?.results?.find(
+    (v) => v.api_key === environmentId,
+  )
+
+  const { data: scheduledData } = useGetChangeRequestsQuery(
     {
-      projectId: `${projectId}`,
+      committed: true,
+      environmentId,
+      live_from_after: date.current,
+      page_size: 1,
     },
-    { skip: !projectId },
+    { skip: !environment },
   )
 
-  const { data: scheduledData } = useGetChangeRequestsQuery({
-    committed: true,
-    environmentId,
-    live_from_after: date.current,
-    page_size: 1,
-  })
-
-  const { data: changeRequestsData } = useGetChangeRequestsQuery({
-    committed: false,
-    environmentId,
-    page_size: 1,
-  })
-
-  const environment = environments?.results?.find(
-    (v) => v.api_key === environmentId,
+  const { data: changeRequestsData } = useGetChangeRequestsQuery(
+    {
+      committed: false,
+      environmentId,
+      page_size: 1,
+    },
+    { skip: !environment },
   )
 
+  if (environmentsLoaded && !environment) {
+    return null
+  }
+
   const changeRequests =
     (Utils.changeRequestsEnabled(
       environment?.minimum_change_request_approvals,