The best high security door lock depends on your door. On a uPVC or composite door, that means a euro cylinder rated TS 007 3-star or Sold Secure SS312 Diamond, fitted at the right length. On a solid timber door, it means a BS 3621 mortice deadlock, usually paired with a British Standard nightlatch. Get those basics right first, then add extra layers where your door is weakest.
This guide covers the seven lock types worth knowing about in the UK, what each rating actually means, and how to pick the right one without paying for features you don’t need.
Quick guide: which lock suits which door?
| Door type | Main lock | Rating to look for |
|---|---|---|
| uPVC (multipoint) | Euro cylinder operating a multipoint lock | TS 007 3-star or SS312 Diamond cylinder |
| Composite (multipoint) | Euro cylinder operating a multipoint lock | TS 007 3-star or SS312 Diamond cylinder |
| Solid timber | 5-lever mortice deadlock plus nightlatch | BS 3621 (or BS 8621 / BS 10621 where keyless exit is needed) |
| Aluminium | Usually a euro cylinder | TS 007 3-star or SS312 Diamond cylinder, not protruding |
If you’re not sure what you’ve got, open the door and look at the edge. A long metal strip with several bolts or hooks is a multipoint lock. A single rectangular faceplate on a wooden door is usually a mortice lock.
Understanding the ratings
Lock packaging is full of claims such as “anti-snap” or “high security”. The marks below are the ones that mean the product has been independently tested.
TS 007 (the star system)
TS 007 is a Kitemark scheme that grades cylinders and door handles with stars. A 3-star cylinder gives the full rating on its own. You can also reach 3 stars by combining a 1-star cylinder with a 2-star security handle. West Yorkshire Police say a 3-star solution is the minimum to aim for when you upgrade.
Sold Secure SS312 Diamond
SS312 is a Sold Secure test for lock cylinders. According to the Master Locksmiths Association, Diamond is the highest grade a cylinder can achieve and is treated as equivalent to a Kitemarked 3-star cylinder. Some cylinders carry both TS 007 3-star and SS312 Diamond.
BS 3621, BS 8621 and BS 10621
These British Standards cover thief-resistant locks, mostly on timber doors. The MLA explains the difference:
- BS 3621: needs a key to open from both sides.
- BS 8621: opens from inside without a key, usually with a thumbturn, so nobody can be locked in.
- BS 10621: a dual-mode lock that combines both functions. It should not be used where the door is the only way out.
Some insurers require locks to BS 3621, but policies vary, so check your own policy wording.
The 7 high security door lock types worth fitting
1. A 3-star anti-snap euro cylinder
The euro cylinder is the part you put your key into on most uPVC and composite doors. Standard cylinders can be attacked by lock snapping, so replacing a basic cylinder with a 3-star or Diamond-rated one is often the best-value upgrade you can make.
Two real examples whose ratings we checked:
- Ultion 3 Star Plus: described on the Ultion website as a 3-star, Police Preferred Specification lock.
- Avocet ABS: a stockist’s product page lists TS 007 3-star Kitemark and Sold Secure Diamond (SS312) certification, but notes that 30mm sizes don’t carry the TS 007 Kitemark.
Other brands make rated cylinders too. Whichever you choose, check that the rating mark is on the cylinder itself and applies to the size you’re buying.
2. A security handle (and a better spindle)
Under TS 007, a 2-star security handle paired with a 1-star cylinder also gives a 3-star result. Security handles shield the cylinder face, which helps where the cylinder would otherwise be exposed.
While you’re changing handles, look at the spindle, the square bar that connects the inside and outside handles. The Security Spindle – HOLDOOR is an 8mm boxed steel spindle, 138mm long and trimmed to fit, with an anti-tamper security clip that resists handle manipulation.
3. A working multipoint lock
Most uPVC and composite doors use a multipoint lock: one key turn throws hooks or bolts into the frame at several points. It only works if you use it properly. Avon and Somerset Police advise always double-locking a uPVC door by lifting the handle, then turning and removing the key.
If the handle is stiff or the key is hard to turn, the door may need adjusting or the gearbox may be worn. Get that fixed rather than leaving the door on the latch. Our guide to weak points of uPVC doors covers what to check.
4. A BS 3621 5-lever mortice deadlock
For solid timber front and back doors, a 5-lever mortice deadlock to BS 3621 is the traditional choice. It sits inside the door edge and throws a deadbolt into a keep on the frame. Look for the Kitemark and “BS 3621” stamped on the faceplate.
Fitting a mortice lock means cutting into the door, so accuracy matters. A poorly cut pocket can weaken the door. Unless you’re confident with woodwork, use a locksmith.
5. A British Standard nightlatch
A rim nightlatch is the surface-mounted lock found on many timber front doors. It latches automatically when the door closes, so it’s convenient but should not be your only lock. Choose one marked BS 3621 if insurance requires it, or BS 8621 if you want to be able to get out quickly without a key. A common set-up is a nightlatch plus a mortice deadlock below it.
6. A restricted key system
With a restricted key system, keys can usually only be cut by authorised dealers, often with proof of ownership. That makes it harder for someone to copy a key without you knowing. These systems tend to cost more, and the key control only works if you keep the paperwork that comes with the lock. If you’ve just moved house or don’t know how many keys are in circulation, replacing the cylinder is the simple fix.
7. A smart lock with a rated mechanical cylinder
Smart locks add keyless entry and activity logs. Physical security still matters most, though. Many smart locks work with or replace a euro cylinder, so check that the cylinder part carries a TS 007 3-star or SS312 Diamond rating, and keep a working key override in case batteries or connectivity fail.
Fitting tips that make a real difference
- Measure carefully. Euro cylinders are sized from the central fixing screw to each end. West Yorkshire Police stress that cylinders must be measured properly and should not be left protruding from the door or handle.
- Test with the door open. Lock and unlock a few times before you close the door.
- Use a qualified locksmith if unsure. The police recommend an MLA-approved locksmith for correct fitting.
- Keep receipts and certificates. They help if your insurer asks what locks you have.
Adding a layer behind the lock
Even a good lock is one line of defence. On a single uPVC back door, a device that holds the handle or door in place adds a second layer that doesn’t depend on the cylinder at all.
- Door Jammer – HOLDOOR: hooks under the inside handle of a uPVC door and braces it 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). It adjusts from 70cm to 120cm, fits most UK single-handle uPVC doors with the handle on the left or right, and needs no drilling or tools. It’s used when you’re at home, because it’s fitted from inside.
- Door Handle Guard – HOLDOOR: a lockable steel cover that drops over the inside lever handle of a single uPVC door and locates in a Secure Keeper screwed to the frame, so the handle can’t be pressed down. It’s currently on pre-order and is being submitted for independent testing, so it has no rating yet.
Neither product is for sliding, French, double or patio doors.
In a fire: find the key, lift the Door Jammer off (or unlock and remove the Door Handle Guard), unlock the door and leave the building.
FAQs
What is the most secure lock for a uPVC door?
A euro cylinder rated TS 007 3-star or Sold Secure SS312 Diamond, sized so it doesn’t stick out, operating a properly adjusted multipoint lock that you lift and lock every time.
Is TS 007 3-star the same as SS312 Diamond?
They are different tests run by different schemes, but both are recognised as top-level cylinder ratings and the MLA treats Diamond as equivalent to a 3-star Kitemarked cylinder.
Do insurers require BS 3621 locks?
Some do, particularly for timber doors. Requirements vary between policies, so check your documents or ask your insurer before you buy.
Can I fit a new euro cylinder myself?
Yes, many people do. Measure carefully, buy the correct size and test it with the door open. If you’re unsure, a locksmith can supply and fit one. See our FAQs for more on uPVC door security.
Next steps
- Check what lock and cylinder your door has now, and look for a rating mark.
- If it’s an unmarked euro cylinder, replace it with a 3-star or Diamond-rated one at the right length.
- Make a habit of lifting the handle and turning the key every time.
- For a single uPVC back door, add a second layer with the Door Jammer – HOLDOOR, which is made in Britain from British steel and costs £90 with free UK delivery, 30-day returns and a 2-year warranty.
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