Merseyside Police said the dogs had ingested the substance at the green space close to Childwall Valley Road, Belle Vale, and were taken to a veterinary surgery at around 2pm on Monday after falling ill.

A fourth dog is also still receiving treatment.

The park area has been cordoned off and people have been told to stay away.

Tests are being carried out to establish what the substance is.

Officers are also scouring CCTV footage, speaking to witnesses and carrying out house-to-house enquiries, the force said.

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Chief Superintendent Jonathan Davies said: "Whilst the incident is obviously of concern I want the public to be reassured that the matter is being dealt with and is under control.

"We would advise people to avoid the area as we seek to establish what the substance is and how it got there.

"Anyone who believes they were in the area and came into direct contact with the substance is advised to seek medical assistance if they believe they have suffered any ill effects.

"We would always advise anyone who comes across an unknown substance to not touch but to report it to police."