A mortice lock is a lock that sits inside a pocket (the mortice) cut into the edge of a door, rather than being screwed to the surface. You will usually see a metal faceplate on the door edge and a keyhole through the face of the door. On UK timber front and back doors, the most common security lock is a five-lever mortice deadlock made to British Standard BS 3621.
This guide explains how mortice locks work, the main types, how to tell what you have, how to measure one for replacement, and what the British Standards mean for security and insurance.
How a mortice lock works
Most traditional UK mortice locks are lever locks. Inside the case is a stack of flat levers. The correct key lifts each lever to exactly the right height, which lets the bolt slide out into a keep cut into the door frame, or back again. A wrong key leaves at least one lever in the way, so the bolt will not move.
Some mortice locks are made to take a euro cylinder instead of a lever key. In that case the cylinder’s security rating matters as much as the lock body.
Why mortice locks matter for security
Because the lock body is buried in the door, there is no exposed case to lever off, and the bolt is thrown deep into the frame. A deadbolt also cannot be pushed back by hand the way a spring latch can, which is one reason deadlocking is so important on external doors.
Police crime-prevention advice, including from West Yorkshire Police, recommends that wooden external doors have a five-lever mortice lock tested to BS 3621, and notes that insurers often require one. Timber and composite doors also commonly pair a British Standard mortice deadlock with a self-locking rim lock, as Avon and Somerset Police describes.
Types of mortice lock
Mortice deadlock
A deadlock has a bolt only, with no latch or handles. It is locked and unlocked with a key. It is often fitted alongside a separate night latch on a front door. The Crime Prevention Website suggests fitting a deadlock about a third of the way up from the bottom of the door.
Mortice sashlock
A sashlock combines a deadbolt with a spring latch operated by handles. You use the handles day to day and the key to deadlock the door. They are common on back doors. The same source suggests fitting a sashlock just below the middle of the door, avoiding the joint with the middle rail.
Mortice latches and bathroom locks
Internal doors often use mortice latches (no lock at all) or bathroom locks with a thumbturn. These are not security locks.
Hook-bolt mortice locks
Sliding doors sometimes use a mortice lock with a hook-shaped bolt that grips the keep, so the door cannot slide open.
Lever count: 3-lever vs 5-lever
More levers generally means more possible key combinations and more resistance to manipulation. Three-lever mortice locks are usually used on internal doors. For external doors, a lock to BS 3621 is the benchmark, and the Master Locksmiths Association explains that a lever mortice lock needs at least five levers to meet it, so a three-lever lock cannot be BS 3621.
British Standards explained
| Standard | What it means | Typical use |
|---|---|---|
| BS 3621 | Thief-resistant lock that needs a key to lock and unlock from both sides | Most houses with more than one exit door |
| BS 8621 | Thief-resistant lock that opens from the inside without a key, usually with a thumbturn | Flats, shared houses and other places where a keyless exit is needed |
| BS 10621 | Dual-mode lock that can be switched so the inside can be locked against keyless opening | Specialist situations, such as some flats with an alternative escape route |
According to the Master Locksmiths Association, BS 3621 requires features such as at least five levers in a lever mortice lock, drill-resistant hardplates and a minimum 20mm bolt throw. Because a BS 3621 lock needs a key from inside, the MLA says it is unsuitable for properties with only one exit door, where a BS 8621 lock may be more appropriate. It also warns that BS 10621 locks should not be used on single-exit homes because of the risk of being trapped.
A BS 3621 lock should carry the British Standard Kitemark on its faceplate. Insurance requirements vary, so check your policy wording rather than assuming. The Crime Prevention Website also suggests getting any confirmation from your insurer in writing.
How to identify a mortice lock on your door
- Look at the door edge. A rectangular metal faceplate screwed flush into the edge, with a bolt (and sometimes a latch) behind it, means a mortice lock.
- Look at the door face. A lever mortice lock has a keyhole shaped for a long-bladed key, often with a small cover plate (escutcheon). A euro cylinder mortice lock has a cylinder in a keyhole-shaped profile.
- Check for markings. The faceplate often shows the maker’s name and, for a BS 3621 lock, the Kitemark and standard number.
- Handles or not? Keyhole only means a deadlock. Keyhole plus handles means a sashlock.
Compare this with a rim lock (such as a night latch), which has a box mounted on the inside face of the door, and a uPVC multipoint lock, which has a long strip down the door edge with several locking points. Most uPVC doors use multipoint locks rather than traditional lever mortice locks; our guide to whether uPVC doors are secure covers those.
How to measure a mortice lock for replacement
Open the door, remove the faceplate screws and carefully slide the lock out. Then measure:
- Case size (depth): from the front of the faceplate to the back of the lock case.
- Backset: from the door edge (where the faceplate sits) to the centre of the keyhole or spindle hole.
- Faceplate: its height and width, and the position of the screw holes.
- For sashlocks, the centres: the distance between the spindle and the keyhole, so your handles line up.
The two most common UK sizes, according to Door Furniture Direct, are:
| Case size | Backset |
|---|---|
| 64mm (2.5 inch) | 44mm |
| 76mm (3 inch) | 57mm |
Some manufacturers list slightly different case depths for the same backsets, so always check the dimensions on the product you are buying rather than relying on the nominal size. Matching your existing lock means it will slot straight into the existing mortice without cutting the door, which is better for the door’s strength.
Write the measurements down and photograph the faceplate and any markings before you order. The Crime Prevention Website notes that doors thinner than 44mm are not suitable for mortice locks, although reinforcing kits can help where a door is only slightly under.
Before you upgrade
- Check the door and frame are sound. A strong lock in a rotten or split door will not hold.
- Make sure the keep in the frame is solid and securely fixed.
- Think about escape. If a BS 3621 deadlock is on your only exit, keep the key somewhere everyone knows, or talk to a locksmith about a BS 8621 lock.
- If you are not confident, a Master Locksmiths Association member can identify, supply and fit the right lock.
For a wider check of your front door, see our guide on how to secure your front door.
FAQs
What is the difference between a mortice lock and a deadlock?
“Mortice” describes how the lock is fitted (inside the door). “Deadlock” describes how it works (a bolt that can only be moved with a key or thumbturn). A mortice deadlock is both.
Is a 5-lever mortice lock the same as BS 3621?
Not automatically. A BS 3621 lever mortice lock must have at least five levers, but not every five-lever lock is tested to BS 3621. Look for the Kitemark and standard number on the faceplate.
Can I fit a mortice lock to a uPVC door?
uPVC doors normally use a multipoint lock rather than a traditional lever mortice lock. Replace like for like and speak to a specialist if you are unsure.
Do insurers require a BS 3621 lock?
Many home insurers ask for one on timber external doors, but requirements vary. Check your policy.
Next steps
- Identify your lock type using the checklist above.
- Measure the case, backset and faceplate before buying a replacement.
- Choose a lock to the right standard for your door: BS 3621 for most houses, or BS 8621 where a keyless exit is needed.
- If your back or side door is a single uPVC door rather than timber, the Door Jammer – HOLDOOR adds a physical layer when you are at home. It hooks under the inside handle and braces the door against the floor, so the door stays shut even if the lock is snapped, bumped or picked. It is Sold Secure Bronze rated and Secured by Design accredited (Police Preferred Specification), adjusts from 70cm to 120cm for most UK single-handle uPVC doors and needs no drilling or tools. Made in Britain from British steel. It is not for sliding, French/double or patio doors. In a fire: find the key, lift the Door Jammer off, unlock the door and leave the building.
- Questions? Read our FAQs or contact HOLDOOR.
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