house spider
Giant house spiders coming to the UKLynette Schimming/Flickr

The giant spiders set to invade UK homes and gardens will grow to as big as 2cm in body length – the top end of their natural range, an expert has said.

Lawrence Bee, from the British Arachnological Association, told IBTimes UK the two species likely to noticeably increase in size are the house spider and garden spider.

"Many of them are fairly common but it depends which part of the country you're in, but generally there are house spiders of one type or another in every part of the UK. The other ones that we're seeing a lot of at the moment is the garden spider, which has a distinctive white cross marking on the back," he said.

"Neither of them are dangerous. They can't actually break the skin. They're not likely to cause a problem."

House spiders have a natural size range from about 17mm to 20mm, while garden spiders can be between 6.5mm and 20mm.

"What we're seeing at the moment is the species being a little bit at the top end of their range of size simply because there's more food, the weather has been warmer and there are more insects about so they can feed more," Bee said.

garden spider
Garden spiders will grow up to 2cm in body lengthTolka Rover/Flickr

"I wouldn't say they are getting bigger as a species; it's just that they're at the top end of their range at the moment rather than their mid or lower part of their size range. Then of course you've got to put the legs on top of that."

Bee explained the spiders are getting bigger because of the warm weather, meaning there has been more food around allowing them to grow bigger.

They are particularly visible at the moment because hatchlings from the start of the year have reached adulthood and are looking for partners to mate with, so are moving around more. "The rest of the year they stay fairly hidden away and looking after themselves," Bee said.

Asked if spiders will revert back to average size next year, he revealed it very much depends on what food is around. "If it's a mild winter then it could be that you'll see more young hatching into young spiders, so there's a likelihood of larger numbers being around and less mortality," Bee said.

"If it's an average year then I would expect spiders – house spiders at least – to be reduced a little bit in size. On a good year, you get big spiders, on a bad year you get smaller spiders. It's fairly straightforward."

In terms of the threat of giant false widow spiders, Bee claimed this should not be a concern. "False widow spiders' danger to us is hugely over-exaggerated anyway," he said. "No we're not talking about false widow spiders this year I think."