New Which? research reveals the dangers posed by cheap fire-risk Christmas tree lights sold by third-party sellers through AliExpress, eBay and Wish. Don't buy these lights, or take them down if you already have, as they could light up your Christmas in all the wrong ways.
Nearly half of the Christmas tree lights that we bought from AliExpress, eBay and Wish were electrically unsafe and dangerous to use.
While more than 90% of those we checked - including four models from Amazon Marketplace - failed to meet the standard allowing them to be lawfully sold in the UK.
Scroll down to watch our video and find out what happens when a Christmas tree catches fire, plus learn what you can do to keep your family and home safe this Christmas.
A genuine fire threat was exposed when we tested two sets of lights bought from sellers based in China. One set of lights was sold through AliExpress, the other through eBay.
When we ran a standard short-circuit test (EN 60598-2-20 Clause 20.13.4, which all safe alarms should pass), the power supply for each set of lights began to smoke and then melt. The insides were charred and the printed circuit board a molten plastic mess by the end of the test.
Bought through eBay - a charred and molten mess following our short-circuit test
Our video shows what could happen if dangerous lights were to set a Christmas tree on fire. Jeremy James, from Royal Berkshire Fire and Rescue Service, tells us that fire could engulf a tree in less than one minute.
In three separate tests, we uncovered sets of Christmas tree lights that could give you an electric shock.
In our electrical strength test, the insulation in one control box broke down, making it a potential electric shock hazard for anyone using it.
Burned-out LEDs following our electrical safety tests
In other tests, when we checked the distances between the live and neutral parts of the circuit board, we found that for three sets of lights the distances were up to half of the legal requirement. In the worst cases, this could lead to arcing, where electricity travels through the air, and an electric shock for anybody touching the lights.
The same three products - bought from AliExpress, eBay and Wish - were so shoddily manufactured that, using minimum force, we were able to pull the cable out of the control box. This again could lead to an electric shock.
Bought from AliExpress - the control box burned and melted during our short-circuit test
Each of the online marketplaces we bought Christmas lights through - so Amazon, AliExpress, eBay and Wish - sold us lights that were either unsafe or didn't meet the compliance standard for lights being sold in the UK.
We run a series of electrical safety tests to find out how safe the Christmas tree lights were. We also checked the pack markings and instructions to see whether they were legal to be sold in the UK.
All of our Christmas tree light tests were carried out by a lab accredited to test to the three relevant standards, EN 60598-2-20, EN 61347 and BS 1363. These cover the testing of strings of lights, the control boxes and the plugs.
We keep all lights in a humid room to see if they continue to work after 48 hours in a damp environment.
We check if the lights can withstand the minimum test voltages required without breaking down or becoming unsafe.
This test is designed to find out if the lights have overload protection. If they don't, they can overheat and could catch fire.
We intentionally short circuit the lights to check that the required fail safes are in place. Products that fail this test could melt or catch fire.
We measure the distances between the live and neutral sections of the circuit board.
We record how much force needs to be used to pull the cable out of the control box. We also check to see whether the control box can withstand impacts without falling to pieces, which could prove dangerous.
We measure the size of the plug pins, look to see that instructions are included and check the CE and WEEE symbols are present. We also make sure that the lights are marked appropriately for where they should be used.
We bought two of the sets of Christmas lights from Argos and John Lewis & Partners to compare high street lights with those you can buy through online marketplaces.
These Argos lights (£8), pictured above, passed all safety tests and compliance checks
The very good news is that both sets of lights from John Lewis & Partners (240 Warm White LED Line Lights with Timer, 24 metres, £15) and Argos (80 Bright White Multi Function LED Lights, £8) passed all of our electrical safety tests.
Every test was passed by these John Lewis & Partners lights (£15)
They were also packaged correctly, came with instructions for use and were appropriately marked for how they should be used.
You won't be able to tell which lights will be safe and which ones could be dangerous just by looking at them. But there are some signs we've seen which could indicate dodgy lights.
Some of the lights that failed our tests or checks came in plain plastic wrapping with no branding.
The six lights pictured above failed our safety tests - they could catch fire or electrocute you
Christmas lights should carry both the CE mark and the WEEE logo. The CE mark shows the manufacturer or importer claiming that the product complies with the EU legislation applicable to the product.
The WEEE symbol shows that the product should be recycled. Two of the sets of lights we tested that were missing one or both of these marks went on to fail key safety tests.
Look for the CE and WEEE marks on the packaging of any lights you're looking to buy this Christmas
Lack of instructions is a bad sign, too. To be lawfully sold in the UK, lights should come with clear instructions.
The worst lights we found in our tests - those where the control boxes melted - cost less than £5 from the online marketplace sellers we bought from.
None of these clues are proof positive that a set of Christmas lights will go on to be dangerous in your home. But if you've bought cheap lights in plain packaging with no instructions, they might well prove to be dangerous.
If you think your lights fit this description, recycle them and buy from a well-known high street store or its online site.
We offered each of the online marketplaces the opportunity to comment on our findings about the dangerous or illegal products being sold through their sites.
AliExpress told us that: 'Customer safety is of paramount importance. We have promptly informed the sellers and removed these listings. Merchants need to comply with all regulations in the markets they sell to.
Amazon said: 'All sellers must follow our selling guidelines and those who do not will be subject to action, including potential removal of their account. The products in question are no longer available.'
An eBay spokesperson told us: 'eBay doesn't permit the listing of unsafe products. The items have been removed and the sellers advised to contact any buyers with the alert and their returns policy.'
Wish said it was working to have these items removed from its platform.
'Tis the season to be jolly and not the season to be electrocuted or worse. So you should think twice before you buy cheap Christmas lights online - based on our tests, the high street is a better and a safer option.
Natalie Hitchins, Which? head of home products and services, said: 'While they might look like the perfect finishing touch for your Christmas tree, many of the cheap and cheerful Christmas lights we've tested from online marketplaces simply aren't safe, and we've found some even have the potential to set a tree alight.
'The significantly better performance of products bought from high street retailers exposes just how necessary it is to strengthen consumer protections for online marketplaces. The new government must make safety a priority and put the legal responsibility on marketplaces to prevent unsafe products from being sold on their sites.'
Which? is campaigning for online marketplaces to have: greater legal responsibility for products sold on their sites, clearer requirements for taking down unsafe products, better enforcement and greater transparency for shoppers about who they're buying from.