Experts say it is hard to tell the difference between toxic and non-toxic varieties.
The Health Protection Agency (HPA) has issued a warning about the dangers of picking poisonous mushrooms.
It says foragers should be aware that it is easy to mix up toxic with non-toxic varieties.
Dozens of people seek medical advice each year after eating wild mushrooms which they have picked themselves.
Some varieties which grow wild in the UK are so poisonous that they can be fatal when eaten. Others cause sickness and severe cramps.
The HPA said foragers should be aware that poisons present in some of the most dangerous mushrooms are generally not destroyed by cooking.
It said that by the end of July this year, its National Poisons Information Service had been consulted for advice on 100 cases.
In 2011 the NPIS saw 257 cases of poisoning linked to eating mushrooms. The numbers were slightly down on the 316 cases seen in 2010, when the weather in late summer and early autumn led to a bumper crop of wild mushrooms.
In 2008, Nicholas Evans, the author of the Horse Whisperer, had to have a kidney transplant after eating the wrong kind of mushroom while on holiday in Moray.
Dr John Thompson, the director of the NPIS unit in Cardiff, said: "The wild mushroom foraging season is under way, which is why we need people to be aware of the potential dangers involved in this activity.
"While many mushrooms growing in the wild are tasty and safe to eat, it is not always easy to differentiate between toxic and non-toxic species - even for people with experience in foraging.
"The NPIS therefore advises that people should not eat mushrooms collected in the wild unless they are familiar with the various species that grow in the UK and are sure that the mushrooms they have collected are safe to eat."
Ray Yantrum from Surrey, who leads expeditions looking for edible mushrooms, said: "My favourite saying is that 100% certainty of identification is only just good enough."
