diff --git a/src/app/base/components/PrefixedInput/PrefixedInput.test.tsx b/src/app/base/components/PrefixedInput/PrefixedInput.test.tsx
index b6984ac984..8a15ef6580 100644
--- a/src/app/base/components/PrefixedInput/PrefixedInput.test.tsx
+++ b/src/app/base/components/PrefixedInput/PrefixedInput.test.tsx
@@ -1,18 +1,24 @@
 /* eslint-disable testing-library/no-node-access */
-import { render, screen } from "@testing-library/react";
+import { render, screen, waitFor } from "@testing-library/react";
 
 import PrefixedInput from "./PrefixedInput";
 
-const { getComputedStyle } = window;
-
 beforeAll(() => {
-  // getComputedStyle is not implemeneted in jsdom, so we need to do this.
-  window.getComputedStyle = (elt) => getComputedStyle(elt);
+  Element.prototype.getBoundingClientRect = vi.fn(() => ({
+    width: 100,
+    height: 0,
+    x: 0,
+    y: 0,
+    top: 0,
+    right: 0,
+    bottom: 0,
+    left: 0,
+    toJSON: vi.fn(),
+  }));
 });
 
 afterAll(() => {
-  // Reset to original implementation
-  window.getComputedStyle = getComputedStyle;
+  vi.restoreAllMocks();
 });
 
 it("renders without crashing", async () => {
@@ -25,18 +31,19 @@ it("renders without crashing", async () => {
   ).toBeInTheDocument();
 });
 
-it("sets the --immutable css variable to the provided immutable text", async () => {
-  const { rerender } = render(
+it("displays the immutable text", async () => {
+  render(
     <PrefixedInput aria-label="Limited input" immutableText="Some text" />
   );
 
-  rerender(
+  expect(screen.getByText("Some text")).toBeInTheDocument();
+});
+
+it("adjusts input padding", async () => {
+  render(
     <PrefixedInput aria-label="Limited input" immutableText="Some text" />
   );
+  const inputElement = screen.getByRole("textbox", { name: "Limited input" });
 
-  // Direct node access is needed here to check the CSS variable
-  expect(
-    screen.getByRole("textbox", { name: "Limited input" }).parentElement
-      ?.parentElement
-  ).toHaveStyle(`--immutable: "Some text";`);
+  await waitFor(() => expect(inputElement).toHaveStyle("padding-left: 100px"));
 });
diff --git a/src/app/base/components/PrefixedInput/PrefixedInput.tsx b/src/app/base/components/PrefixedInput/PrefixedInput.tsx
index 4454a43097..b391baf251 100644
--- a/src/app/base/components/PrefixedInput/PrefixedInput.tsx
+++ b/src/app/base/components/PrefixedInput/PrefixedInput.tsx
@@ -1,5 +1,4 @@
-import type { RefObject } from "react";
-import { useEffect, useRef } from "react";
+import { useLayoutEffect, useRef } from "react";
 
 import type { InputProps } from "@canonical/react-components";
 import { Input } from "@canonical/react-components";
@@ -9,45 +8,42 @@ export type PrefixedInputProps = Omit<InputProps, "type"> & {
   immutableText: string;
 };
 
-// TODO: Upstream to maas-react-components https://warthogs.atlassian.net/browse/MAASENG-3113
 const PrefixedInput = ({ immutableText, ...props }: PrefixedInputProps) => {
-  const prefixedInputRef: RefObject<HTMLDivElement> = useRef(null);
+  const prefixTextRef = useRef<HTMLDivElement>(null);
+  const inputWrapperRef = useRef<HTMLDivElement>(null);
 
-  useEffect(() => {
-    const inputWrapper = prefixedInputRef.current?.firstElementChild;
-    if (inputWrapper) {
-      if (props.label) {
-        // CSS variable "--immutable" is the content of the :before element, which shows the immutable octets
-        // "--top" is the `top` property of the :before element, which is adjusted if there is a label to prevent overlap
-        inputWrapper.setAttribute(
-          "style",
-          `--top: 2.5rem; --immutable: "${immutableText}"`
-        );
-      } else {
-        inputWrapper.setAttribute("style", `--immutable: "${immutableText}"`);
-      }
-
-      const width = window.getComputedStyle(inputWrapper, ":before").width;
+  useLayoutEffect(() => {
+    const prefixElement = prefixTextRef.current;
+    const inputElement = inputWrapperRef.current?.querySelector("input");
 
+    if (prefixElement && inputElement) {
       // Adjust the left padding of the input to be the same width as the immutable octets.
       // This displays the user input and the unchangeable text together as one IP address.
-      inputWrapper
-        .querySelector("input")
-        ?.setAttribute("style", `padding-left: ${width}`);
+      const prefixWidth = prefixElement.getBoundingClientRect().width;
+      inputElement.style.paddingLeft = `${prefixWidth}px`;
     }
-  }, [prefixedInputRef, immutableText, props.label]);
+  }, [immutableText, props.label]);
 
   return (
-    <div className="prefixed-input" ref={prefixedInputRef}>
-      <Input
-        className={classNames("prefixed-input__input", props.className)}
-        type="text"
-        wrapperClassName={classNames(
-          "prefixed-input__wrapper",
-          props.wrapperClassName
-        )}
-        {...props}
-      />
+    <div
+      className={classNames("prefixed-input", {
+        "prefixed-input--with-label": !!props.label,
+      })}
+    >
+      <div className="prefixed-input__text" ref={prefixTextRef}>
+        {immutableText}
+      </div>
+      <div ref={inputWrapperRef}>
+        <Input
+          className={classNames("prefixed-input__input", props.className)}
+          type="text"
+          wrapperClassName={classNames(
+            "prefixed-input__wrapper",
+            props.wrapperClassName
+          )}
+          {...props}
+        />
+      </div>
     </div>
   );
 };
diff --git a/src/app/base/components/PrefixedInput/_index.scss b/src/app/base/components/PrefixedInput/_index.scss
index aa57e83f3a..ab7969f9a4 100644
--- a/src/app/base/components/PrefixedInput/_index.scss
+++ b/src/app/base/components/PrefixedInput/_index.scss
@@ -2,15 +2,18 @@
   .prefixed-input {
     position: relative;
 
-    &__wrapper::before {
+    .prefixed-input__text {
       position: absolute;
       pointer-events: none;
       padding-left: $spv--small;
       padding-bottom: calc(0.4rem - 1px);
       padding-top: calc(0.4rem - 1px);
-      // TODO: Investigate replacement for using these variables https://warthogs.atlassian.net/browse/MAASENG-3116
-      content: var(--immutable, "");
-      top: var(--top, "inherit");
+    }
+
+    &--with-label {
+      .prefixed-input__text {
+        top: 2.5rem;
+      }
     }
   }
-}
+}
\ No newline at end of file
diff --git a/src/app/base/components/PrefixedIpInput/PrefixedIpInput.test.tsx b/src/app/base/components/PrefixedIpInput/PrefixedIpInput.test.tsx
index cc6c9e8024..41042b8f40 100644
--- a/src/app/base/components/PrefixedIpInput/PrefixedIpInput.test.tsx
+++ b/src/app/base/components/PrefixedIpInput/PrefixedIpInput.test.tsx
@@ -10,18 +10,6 @@ import PrefixedIpInput from "./PrefixedIpInput";
 
 import { renderWithBrowserRouter } from "@/testing/utils";
 
-const { getComputedStyle } = window;
-
-beforeAll(() => {
-  // getComputedStyle is not implemeneted in jsdom, so we need to do this.
-  window.getComputedStyle = (elt) => getComputedStyle(elt);
-});
-
-afterAll(() => {
-  // Reset to original implementation
-  window.getComputedStyle = getComputedStyle;
-});
-
 it("displays the correct range help text for a subnet", () => {
   render(
     <Formik initialValues={{}} onSubmit={vi.fn()}>
@@ -31,20 +19,6 @@ it("displays the correct range help text for a subnet", () => {
   expect(screen.getByText("10.0.0.[1-254]")).toBeInTheDocument();
 });
 
-it("sets the --immutable css variable to the immutable octets of the subnet", () => {
-  render(
-    <Formik initialValues={{}} onSubmit={vi.fn()}>
-      <PrefixedIpInput aria-label="IP address" cidr="10.0.0.0/24" name="ip" />
-    </Formik>
-  );
-
-  // Direct node access is needed here to check the CSS variable
-  expect(
-    screen.getByRole("textbox", { name: "IP address" }).parentElement
-      ?.parentElement
-  ).toHaveStyle(`--immutable: "10.0.0."`);
-});
-
 it("displays the correct placeholder for a subnet", () => {
   render(
     <Formik initialValues={{}} onSubmit={vi.fn()}>