Hartlepool Council believes three organisations may be responsible and has written to them about the problem, in Seaton Carew.

In law they have until September to respond to the allegations.

It is estimated the clear-up could cost �4m. Areas affected are Wainwright Walk, Wainwright Close, Hornby Close, Cowley Close and Lithgo Close.

The problem was discovered last October when tests on soil found higher levels than normal of arsenic, lead and zinc in soil.

Chemically-treated

A council spokesman said officials wanted to track down who is responsible for the contamination so they could help cover the cost of the clear-up.

He said: "There was never going to be a quick fix solution to this situation. We believe three organisations may be responsible for the contamination.

"But we are not saying who at this stage as we have written to them and they have until September to respond.

"We are anxious to resolve this situation as soon as possible."

The council has sent out letters to residents explaining that the land was previously used as a storage area for chemically-treated pit-props, which are large blocks of wood used to support mine roofs.