An investigation is under way after at least three dogs died and more fell ill after visiting a woodland area, Natural England said today.
The inquiry is looking at whether the use of illegal chemicals in the Sherwood area of Nottinghamshire could be responsible for the deaths, according to the Forestry Commission, which owns the land.
Warning signs have been put up in the woods in Haywood Oaks, notifying dog-owners that animals have fallen ill and advising them to keep their pets on leads.
A spokeswoman for Natural England said: "We have received reports that three dogs and one ferret have died and a further five dogs have been taken ill after being in the vicinity of the woodlands, since the beginning of September.
"We are currently awaiting results of tests to clarify the cause."
There have been no reports of humans falling ill.
It is thought more likely that the deaths resulted from something in the water that occurred naturally, or something transmitted by a mite or tick than from the use of chemicals.
A spokesman for the Forestry Commission said: "We'd be very concerned if illegal chemicals were being used."