An old landfill in the West Midlands thought to contain toxic waste may be leaking into a Unesco World Heritage Site, local Green Party members have said.

Stoneyhill is in the south-east of Telford, Shropshire, above the Ironbridge Gorge, which is famed for being one of the birthplaces of the Industrial Revolution in the 18th century.

Campaigners believe hazardous waste was dumped there carelessly by the now-defunct chemical manufacturer Monsanto and they are concerned about its contents leaking into the environment as well as the proximity of a new housing estate being built nearby.

Monsanto has racked up billions in liability for pollution in the United States but has never paid out any compensation in the UK, despite records showing it produced tens of thousands of tonnes of chemical waste here which were then dumped in unregulated landfills across England and Wales.

Environment Agency (EA) data shows Stoneyhill received unspecified industrial, commercial, household and special – meaning hazardous – waste as well as liquid sludge between February 1985 and March 1991.

Reverend Paul Cawthorne, who has spent years investigating Monsanto dump sites, said he saw a “2ft-high pile” of records in an EA office 20 years ago showing the source of Stoneyhill’s waste, but that the EA is now telling him those records do not exist.

Asked whether it possesses any such records and if it is investigating the possible pollution, the EA refused to comment.

Rev Cawthorne also said the local council has photographs proving that barrels of sludge were brought from Monsanto’s factory in Ruabon, North Wales, and carelessly dumped in the mud.

He shared copies of these photographs with the PA news agency but they cannot be published for copyright reasons.

They show rusted barrels rolling off the back of a lorry, crusty brown sludge spilling on to the ground, and other barrels buried haphazardly in the mud.

Other photographs show notes on the back with one labelled “Stoneyhill” and dated from 1992 reading: “Area at poor cover/empty drum marked hazardous.”

Another says: “Monsanto load deposited at Stoneyhill.”

The photographs could not be independently verified by PA.

Telford and Wrekin Council said chemical waste had come from Monsanto but that EA records showed it to be “not toxic”.

Officials also said there is “routine testing” of the landfill and surrounding waters which has found “no evidence that leachate is leaving the site in an uncontrolled manner”, although the most recent test was in January.

Members of Telford and Wrekin Green Party believe pollution has occurred more recently, with liquid seeping from underneath the fence at the border of the landfill.

One member, who wished to remain anonymous, said it could be running down a hill towards the world-famous Ironbridge Gorge.

They also described a “strange smell about the place”, but they said a proper investigation is needed to identify any potential contamination.

Pat McCarthy, co-ordinator for the Telford and Wrekin Green Party, said he has reported the possible pollution to the EA but it has not responded.

He said: “There are so many unknowns about it, but that in itself is the problem.

“There is a need to clarify what’s in the ground and what dangers it poses to the local residents, wildlife and waterways.

“Even worse, there seems to be a policy of denial and cover-up with both the agencies and the local authority. Telford has a particular issue about the way it deals with community concerns.

“There seems to be a culture here of feeling threatened by the community and there also seems to be this centralised control with the council and so they are not interested in allowing the community a voice.”

Rev Cawthorne also accused the local authority of not being honest with the public, adding: “Telford Council have shown a remarkable lack of candour about what is in the toxic waste site, close to where houses are now being built. This seems unfair to prospective house purchasers.

“I was shown a 2ft-high pile of tipping records of what was dumped at Stoneyhill in the Environment Agency’s Shrewsbury office. Where are those now? I think we should be told.”

Telford and Wrekin Council said details of the landfill are part of the planning process and that it is “clear with residents and the wider public that this is an actively managed closed landfill site and complies with rules set by Government”.