Is Undertaking Illegal in the UK?

If you’ve ever found yourself stuck behind someone stubbornly sitting in the middle lane of the motorway, you’ve probably thought: "Can I just pass them on the left?"

That’s called undertaking – and while it might feel like the quickest way past a slow driver, the rules around it aren’t as straightforward as you might think.

Here’s what the law says, when undertaking is allowed, and when it could land you in trouble.

What is undertaking?

Undertaking is when you pass another vehicle on its left-hand side, usually on a multi-lane road like a dual carriageway or motorway.

In the UK, we normally overtake on the right, so passing on the left goes against the flow of expected driving behaviour — which is why it’s not usually encouraged.

What does the Highway Code say?

The Highway Code (Rule 268) puts it pretty clearly:

“Do not overtake on the left or move to a lane on your left to overtake.”

But there are some exceptions, and that’s where things get a bit more flexible.

When is undertaking allowed?

There are a few situations where undertaking is permitted — and perfectly legal.

1. In slow-moving traffic

If you’re on a busy motorway or dual carriageway and all lanes are crawling along, it’s common for traffic in one lane to move slightly faster than others. If you happen to pass a car on the right without switching lanes, that’s considered part of normal traffic flow — not illegal undertaking.

2. When road signs or markings direct you to

Certain junctions, roundabouts, or filter lanes are designed so that vehicles must pass on the left. In those situations, undertaking is allowed and expected.

3. When traffic is clearly moving in separate lanes

Some roads have lane-specific directions — for example, one for turning left and one for going straight. If you’re in the left lane and the right lane is moving more slowly, passing the cars to your right isn’t considered undertaking in the illegal sense.

When is undertaking illegal?

Undertaking becomes a problem when it’s done deliberately and aggressively. For example:

• You’re speeding past someone in the middle or outside lane using the left lane, just because they’re slower

• You’re weaving between lanes to get ahead

• You’re passing on the left when the right-hand lane is clearly open

In these cases, undertaking could be classed as careless or dangerous driving. That can mean:

• Fines

• Penalty points on your licence

• In more serious situations, a court appearance or driving ban

Even if no one gets hurt, it can still be considered unsafe if it causes other drivers to brake or swerve suddenly.

What about middle-lane hoggers?

We all know how frustrating it is to see a car happily cruising in the middle lane with the left lane wide open. It’s not just annoying — it’s actually an offence under careless driving laws.

But that doesn’t give the green light to undertake. The best thing to do is:

• Wait until it’s safe to pass on the right

• Avoid tailgating or flashing lights

• Let them be the one breaking the rules, not you

If it’s genuinely dangerous or disruptive driving, you can report it to the police with dashcam footage.

The bottom line

Undertaking isn’t always illegal — but it’s rarely the safest choice. There are situations where it’s allowed, like in slow-moving traffic or where signs make it necessary. But if you’re doing it just to get around someone who's not paying attention, you could end up on the wrong side of the law.

As a general rule? Stick to overtaking on the right. It’s safer, clearer, and keeps you in line with the Highway Code.

Got more driving questions or thinking about your next lease car? We’re always happy to help — just give the Motorlet team a shout.