According to the Daily Star, multiple sightings of the black widow's cousin have been reported across southern parts of the UK, but there has been a sighting as far up as Birmingham.
The Steatoda noblis spiders, the most poisonous in Britain, originate from the Canary Islands. They have a distinctive marking on their back which looks like a white skull, and only usually bite if they feel threatened.
If bitten, most people will suffer localised pain and swelling after a bite, but in extreme cases it could trigger an allergic reaction known as anaphylactic shock, which can be fatal.
Most sightings have been in woody campsites, sheds, outbuildings, gardens and parks, but many fear they will move inside houses for warmth over winter.
Ricki Whitmore from Essex nearly lost his leg after being bitten by a false widow while working at a school. He saw a huge nest of them, which he described as "creepy", reports the Daily Star.
After trying to sweep them aside, one of the creatures leapt on him and bit his thigh.
By the time he got home, his leg had swollen to twice its normal size and he was rushed to hospital. He was told he needed an operation to remove the poison, or face losing his leg.
He has lost a large portion of muscle from his thigh, and all need six months of physiotherapy to be able to walk again. But he managed to joke that he would not be holidaying in the Canary Islands, where the spiders came from, anytime soon.
Steve Harris, 22, from Dawlish, Devon, had to undergo an emergency operation at Torbay Hospital after being bitten, which left him with an open wound needing surgeons to cut away the poison.
Mr Harris went to the hospital four days after being bitten after the bite "grew and grew" and "turned black". He told This Is South Devon: "I went to Torbay Hospital and they instantly said I had been bitten by a spider – they said they had seen six other people attending with similar bites within the previous week. They told me the false black widow spider was the culprit.
"They operated on me immediately and it took half an hour to cut away the area around the bite to get at the poison. I now have an open wound and have to wait for it to heal over.
"I was in agony. I have never had pain like that before in my life. It's still very painful now. I still can't sleep properly and find it virtually impossible to get in and out of a car."
The false black widow has long been a resident of Devon, arriving from the Canaries in the 1870s on boats carrying bananas.
But recent climate change has seen the population spread across the south of England, and now it could be moving further north.
It is about the size of a 50p piece, and people who are bitten have reported burning pain, swelling and a sick feeling. The bite also often turns black or yellow.