Around half a pint of the liquid metal was found spread over a small area of wasteland behind Moorlands School, Winchester Road, West Bromwich.

The children were checked by a consultant at Sandwell Hospital before being released.

An elderly woman took bottles believed to have contained the metal away from the children. She was later tracked down by police officers who carried out house to house inquiries in the area.

A spokeswoman for West Bromwich police said two empty containers had been taken away for further examination.

Police Inspector Darren Brook said the situation could have been much worse.

The school's head teacher, Ben Smith, said the area had been cordoned off and searched by police. He warned children to take care.

Councillor Bill Thomas, chair of the Sandwell Council education committee, said: "As a former chemistry teacher I am very much aware of the potential risks of handling mercury, which has long been on the list of dangerous substances and should be handled with care.

"I sincerely hope there will be no ill effects for the young people concerned. We need to have strict precautions in place to ensure that young people cannot come into contact with this substance."

A Sandwell Council spokeswoman said it was unclear how the mercury had got into the hands of the children.

Permanent nerve damage

Mercury can cause serious and permanent nerve and kidney damage.