Is it legal to overtake on the left on a motorway?

It's tempting to 'undertake' when confronted with middle lane morons, but the law is clear - only in slow moving traffic

No.

But let's look at it from the real world perspective. You're on the motorway, doing a steady 70mph in the left-hand lane like you're supposed to. Ahead, you see it. The classic British roadblock: someone camped out in the middle lane, doing 60mph with a clear road in front of them. The outside lane is busy with cars overtaking, and you’re stuck. The temptation is huge, isn't it? That wide-open inside lane looks like a personal invitation to just nip past and get on with your day.

But you hesitate. A faint memory of your driving instructor's voice echoes in your head. "Never, ever overtake on the left." But is that always true? What about in a traffic jam when your lane is moving but the one next to you has ground to a halt? Let's cut through the confusion and get a straight answer on whether 'undertaking' will land you in hot water.

The Rulebook vs. The Real World

The first place to look is the Highway Code, that book we all crammed for our theory test and then promptly forgot most of. The main rule, Rule 267, is pretty clear-cut: "Overtake only on the right." That’s the golden rule of motorway driving. You stay left, you move into the middle or outside lanes to overtake, and then you move back left again. Simple.

But the clever clogs who write the rules know that motorways aren't always a perfect, free-flowing paradise. They know all about traffic jams. That's where Rule 268 comes in, and it's the bit that causes all the debate. It starts by saying, "Do not overtake on the left or move to a lane on your left to overtake." So, no deliberately weaving into the inside lane to get past someone.

However, it then gives a massive 'but'. It says that when things get congested and traffic is moving slowly in all lanes, you're allowed to "keep up with the traffic in your lane even if this means passing traffic in the lane to your right."

This is the key difference right here. The rulebook makes a distinction between 'overtaking' and 'passing'.

Overtaking is an active move. You see a slower car, you check your mirrors, indicate, and deliberately move into another lane to get past them. On a motorway, this should only ever be a move to the right.
Passing is passive. You're just sitting in your lane in slow-moving, queuing traffic. Your lane just happens to be moving a bit faster than the lane to your right. You’re not expected to slam on your brakes to avoid inching past them. You can just go with the flow.

So, if you're stuck in that classic rush-hour crawl, and the middle lane is at a standstill while your lane is chugging along at 15mph, you are perfectly entitled to pass them on the left. You're not undertaking; you're just keeping up with your lane.

So, Why is Undertaking So Dangerous?

If passing on the left is fine in a jam, why is it such a cardinal sin when traffic is flowing freely? It all comes down to basic expectations and blind spots.

From the moment we get behind the wheel, we're trained that the danger—the faster-moving traffic—will come from the right. All our instincts, our mirror checks, our "lifesaver" glances over the shoulder, are geared towards what's happening on our offside.

A car suddenly appearing on your left is a genuine surprise. Most cars have a nasty blind spot over the driver’s left shoulder, which is much larger than the one on the right. If someone in the middle lane finally wakes up and decides to move back to the left just as you're zipping past on the inside, they might not see you until it's too late. The result is often a nasty side-swipe collision that could easily have been avoided.

This brings us to the main reason we all want to undertake in the first place: the middle-lane hogger. It's infuriating. They effectively block the motorway, forcing everyone to squeeze into the outside lane to get past. While every fibre of your being might be screaming to just fly past them on the inside, the law says you can't. The correct, and admittedly long-winded, procedure is to treat them as a stationary obstacle. You must move from lane one, across lane two, and into lane three to overtake them, before moving all the way back to lane one again. It feels mad, but it’s the safe and legal way to do it.

What's the Worst That Can Happen?

Let's say you ignore the rules and undertake someone. Best case scenario? Nothing. You get away with it. But if a police officer spots you, or if it causes an accident, you could be in trouble.

Undertaking isn't a specific offence in itself, but it's often classed as "careless driving" or "driving without due care and attention." This is a catch-all term for poor, selfish, or inconsiderate driving. If you're pulled over, you could be looking at a Fixed Penalty Notice, which usually means a £100 fine and three points on your licence.

If your undertaking manoeuvre is particularly aggressive or leads to a crash, you could find yourself in court facing much more serious penalties, including a much larger fine or even having your licence taken away. It's a hefty price to pay for gaining a few seconds on your journey.

The Bottom Line for the Everyday Driver

Let's boil it all down to a simple guide you can actually use:

Free-flowing traffic? Overtake on the right. Always. No exceptions, not even for the world's most annoying middle-lane hogger.
Stuck in a jam? If everyone is crawling along and your lane starts moving a bit faster than the one on your right, relax. You can 'pass' them. Don't weave about, just stay in your lane and go with the flow.
Think about why. The rule isn't there just to annoy you. It's because drivers aren't expecting you on their left. It’s about keeping everyone safe, including you.
Be predictable. The best way to drive on a motorway is to be smooth and predictable. Stick to your lane, indicate early, and only use the right-hand lanes for overtaking.