Arachnophobes have received an urgent warning after new research revealed thousands of “rat-sized” spiders have invaded the UK.

Fen Raft Spiders, which are around the same size as a man’s hand, was once confined to three sites in Suffolk and Sussex.

However, a repopulation campaign has seen its number soar to 3,750 breeding females at 12 sites in Norfolk alone.

The spider, which spins webs as big as pizzas, was on the verge of extinction as recent as 2010.

But concerted efforts to improve the creatures’ habitat have helped boost numbers.

Damp weather this year has also proved integral to the surge, adding a natural protection to the spider’s preferred habitat.

Tim Strudwick, RSPB Mid Yare nature reserves site manager, said he was proud that the spider had recovered.

Strudwick said: “The Fen Raft Spider is one of the UK’s rarest invertebrates and we are proud of the role our reserves and teams have played in its recovery.

“These spiders have an important role in maintaining the rich aquatic diversity found in the grazing ditches on our reserves.

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“The females are impressive in size, but also beautiful – they are truly special to see.”

Fen Raft Spiders, whose diet includes other spiders, damselflies, dragonfly larvae and pond skaters, can catch tadpoles and even fish.

The creature is known for being semi-aquatic and can even run across water.

It is easily spotted through its size and the distinctive white or cream stripes running along the side of the abdomen.

Fen Raft Spiders remain vulnerable and listed as a Priority Species.

However, the annual spider surveys take place in September in what many hope will be another successful breeding year.

Strudwick also revealed that the spiders can be seen in at the RSPB’s other Norfolk site in Strumpshaw Fen.

He added: “The spiders are only seen in the grazing marsh ditches and are fairly shy but are easier to see from June to September.

“Walking along the meadow trail gives visitors at Strumpshaw Fen the best chance of sightings of the mature females and their glistening nursery webs, so now is the perfect time to make a trip to the reserve.”

Jane Sears, an RSPB ecologist who was involved in Fen Raft Spider conservation, also said: “The RSPB has played an important role in the reintroduction of these spiders and means we must continue to restore, manage and protect the wetland habitats where the Fen Raft Spider thrives to ensure the future of not just this species, but many others too.”